staging: comedi: adl_pci8164: simplify axis register determination

The low-level i/o functions in this driver simply read/write a
register based on the channel in insn->chanspec and an offset.

Create a macro, PCI8164_AXIS(), that takes the channel number as
a parameter and returns the register value.

Remove the switch() statements used to figure out the 'axis_reg'
and use the new macro instead.

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>