staging: comedi: plx9080.h: define PLX_<REG>_TO_<FIELD>(r) macros

Various macros in "plx9080.h" take the form `PLX_<REG>_<FIELD>(x)`,
where `<REG>` is a register name, `<FIELD>` is a field within the
register, and `x` is a value for the field specified by the caller.  The
macros construct a partial register register with the specified field
value placed in the appropriate bits of the register value, and other
bits of the register value zeroed.  Add corresponding macros of the form
`PLX_<REG>_TO_<FIELD>(r)` that extract a field value from a specified
register values.  Remove macros of the form `PLX_<REG>_<FIELD>_SHIFT`
that specified the bit position of a field within a register value as
they are no longer useful.

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