staging: comedi: addi_apci_3120: tidy up timer_mode masking

Define a macro that returns the mask of the timer_mode bits for a given
timer. Use the macro to remove the "magic" values used to clear the bits.

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>