staging: comedi: hwdrv_apci3501: remove useless read/mask to stop watchdog

The watchdog is stopped in apci3501_write_insn_timer() by writing a 0 to
the timer control register. There is no need to read the register first
and mask it (as done when the timer is used as a timer).

Reported-by: coverity (CID 1227052)
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>