cris: fasttimer: fix mixed declarations and code compile warning

arch/cris/arch-v32/kernel/fasttimer.c: In function ‘timer_trig_handler’:
arch/cris/arch-v32/kernel/fasttimer.c:353:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  fast_timer_function_type *f;
  ^

Signed-off-by: Niklas Cassel <nks@flawful.org>
Signed-off-by: Jesper Nilsson <jespern@axis.com>