m68knommu: fix 68328 local setting of timer interrupt handler

Compiling for 68328 based targets fails with:

    arch/m68k/platform/68328/timers.c: In function ‘hw_tick’:
    arch/m68k/platform/68328/timers.c:65:2: error: implicit declaration of function ‘arch_timer_interrupt’
    arch/m68k/platform/68328/timers.c: At top level:
    arch/m68k/platform/68328/timers.c:102:6: error: conflicting types for ‘hw_timer_init’
    arch/m68k/include/asm/machdep.h:36:13: note: previous declaration of ‘hw_timer_init’ was here

Changes made to hw_timer_init() didn't get updated in the 68328 timer code.
So process and call the "handler" arg that is now passed into that
hw_timer_init() function.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>