tls: tidy up recently added ECDSA code

tls: tidy up recently added ECDSA code

function                                             old     new   delta
tls_handshake                                       1935    1930      -5
static.OID_ECDSA_KEY_ALG                              21      11     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-15)             Total: -15 bytes
   text	   data	    bss	    dec	    hex	filename
 950036	    477	   7296	 957809	  e9d71	busybox_old
 950048	    477	   7296	 957821	  e9d7d	busybox_unstripped

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