brctl: simplify str_to_jiffies()

brctl: simplify str_to_jiffies()

function                                             old     new   delta
write_uint                                             -      96     +96
brctl_main                                           721     678     -43
write_ulong                                           96       -     -96
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(add/remove: 1/1 grow/shrink: 0/1 up/down: 96/-139)           Total: -43 bytes

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