staging: comedi: adl_pci9118: introduce pci9118_ai_reset_fifo()

Introduce a helper function to reset the analog input FIFO. For
aesthetics, rename the register used to reset the FIFO.

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>