staging: comedi: addi_apci_3120: introduce apci3120_ai_reset_fifo()

A dummy read of APCI3120_TIMER_MODE_REG resets the analog input FIFO. Introduce
a helper function to clarify this.

It's not necessary to do a dummy read of the FIFO (base + 0) before reseting it.
Remove the unnecessary dummy reads.

The APCI3120_TIMER_MODE_REG is a 16-bit register. This fixes a couple 8-bit reads.

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>