staging: comedi: addi_apci_3120: tidy up APCI3120_ENABLE_TIMER[012]

For aesthetics, replace these defines with a macro that returns the
correct bit needed to set the gate bit to enable a timer.

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>