staging: comedi: addi_apci_3120: fix timer 2 disable in apci3120_write_insn_timer()

The wrong define is being used to disable the gate to stop timer 2 in this
function.

Use the apci3120_timer_enable() helper to properly disable the 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>