Update h8_adc.c

Fixes this on MSVC2010:

src\emu\cpu\h8\h8_adc.c(499) : error C4716: 'h8_adc_2655_device::get_channel_index' : must return a value
make: *** [obj/vwindowsd/emu/cpu/h8/h8_adc.o] Error 2