staging: comedi: hwdrv_apci3501: refactor apci3501_config_insn_timer()

The handling for the watchdog and timer modes is very similar. Refactor
this function to use a common code path for both 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>