staging: comedi: hwdrv_apci3501: refactor apci3501_write_insn_timer()

The handling of the ADDIDATA_WATCHDOG and ADDIDATA_TIMER is identical
except for the "stop" operation. Refactor this function to use a common
code path for both timer modes.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>