tls: avoid using int16 in pstm code

tls: avoid using int16 in pstm code

function                                             old     new   delta
pstm_div                                            1472    1522     +50
psRsaEncryptPub                                      403     413     +10
pstm_2expt                                            91      96      +5
pstm_clear                                            68      72      +4
pstm_init                                             39      42      +3
pstm_unsigned_bin_size                                36      37      +1
pstm_montgomery_reduce                               398     399      +1
pstm_init_size                                        45      46      +1
pstm_zero                                             39      38      -1
pstm_set                                              35      34      -1
pstm_read_unsigned_bin                               112     109      -3
pstm_mulmod                                          123     120      -3
pstm_mod                                             116     113      -3
pstm_cmp                                              57      54      -3
pstm_sub                                             107     102      -5
pstm_to_unsigned_bin                                 157     151      -6
pstm_clamp                                            63      57      -6
pstm_add                                             116     108      -8
pstm_grow                                             81      72      -9
pstm_count_bits                                       57      48      -9
pstm_init_copy                                        84      72     -12
pstm_cmp_mag                                          93      78     -15
pstm_sqr_comba                                       567     551     -16
pstm_montgomery_calc_normalization                   158     140     -18
pstm_copy                                            115      92     -23
pstm_lshd                                            133     109     -24
pstm_mul_comba                                       525     500     -25
pstm_mul_d                                           251     224     -27
s_pstm_sub                                           256     228     -28
s_pstm_add                                           370     337     -33
pstm_div_2d                                          444     409     -35
pstm_mul_2                                           195     156     -39
pstm_rshd                                            154     104     -50
pstm_mul_2d                                          247     186     -61
pstm_exptmod                                        1524    1463     -61
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 8/27 up/down: 75/-524)         Total: -449 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>