staging: comedi: drivers: rename PLX PCI 9080 register offsets

Rename the macros in "plx9080.h" that define the offsets of registers,
following the pattern `PLX_REG_<NAME>`, where `<NAME>` is the register
name from the PLX PCI 9080 Data Book.

Add defines for the "Mailbox" registers, and add parameterized macros
for the mailbox registers and the DMA control registers.  Make use of
the parameterized versions of the macros where it seems appropriate.

The registers for supporting the I2O (Intelligent Input/Output) feature
are largely left undefined, just defining enough to allow the I2O
feature to be disabled.

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