staging: comedi: drivers: add PLX PCI 9080 DMATHR register values

Add macros in "plx9080.h" that define values for the DMATHR register
values.  Use the prefix `PLX_DMATHR_` 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>