staging: comedi: adl_pci9118: tidy up analog input fifo register define

For aesthetics, rename the define used for the analog input fifo register
and tidy up its use in the driver.

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>