staging: comedi: s626: bitfield manipulation macros for CRA, CRB and setup

Some bits of the 'CRB' register have different functions when read or
written, so add macros to define the read-only parts.

Add macros to define the widths of the bitfields in the 'CRA' and 'CRB'
registers and the standard encoder setup value.

Add macros to construct and extract parts of the 'CRA' and 'CRB'
register values and the standard encoder setup value, along with a
couple of general helper macros for the above.

Redefine the bitfield mask macros for 'CRA', 'CRB' and standard encoder
setup using the above.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>