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

Replace the existing macros in "plx9080.h" that define values for the
DMADPR0 and DMADPR1 registers.  (A little-endian version of the register
value is also placed in the `next` member of `struct plx_dma_desc`.)
Use the prefix `PLX_DMADPR_` 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>