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

Replace `enum bigend_bits` in "plx9080.h" with a bunch of macros
defining values for the BIGEND register.  Use the prefix `PLX_BIGEND_`
for the macro names.  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>