staging: comedi: adl_pci9111: cleanup ai read in pci9111_ai_insn_read()

The shift, maxdata, and invert values, used to handle the 12-/16-bit
analog input differences, can be calculated based on the subdevice
maxdata value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>