staging: comedi: drivers: re-do PLX PCI 9080 MARBR register values

Replace `enum marb_bits` in "plx9080.h" with a bunch of macros defining
values for the MARBR and DMAARB registers (which are the same
Mode/Arbitation register at two different offsets).  Use the prefix
`PLX_MARBR_` for the macros.  Make use of the `BIT(x)` and
`GENMASK(h,l)` macros to define the values.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>