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

Replace the existing macros in "plx9080.h" that define values for the
DMCFGA register.  Use the prefix `PLX_DMCFGA_` 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>