staging: comedi: addi_apci_3120: introduce apci3120_timer_enable()

Introduce a helper function to enable/disable a timer.

The disable of timers 0 and 1 in apci3120_interrupt() is probably not needed.
For now use the helper function to make sure the devpriv->ctrl bits are cleared
correctly to disable the timers.

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>