staging: comedi: addi_apci_3120: introduce apci3120_clr_timer2_interrupt()

A dummy read of APCI3120_CTR0_REG clears the timer 2 interrupt. Introduce a
helper function to clarify this.

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>