staging: comedi: s626: make CRA and CRB setup conversions more readable

Use the new macros defined in "s626.h" for constructing and decomposing
'CRA', 'CRB' and standardized encoder setup values to make the
conversions between standardized encoder setup values, and CRA/CRB
register values easier to follow.

There is some messing about with the 'IndxSrc' values which are 1-bit
wide in the standardized encoder setup, and 2-bit wide in the 'CRA' and
'CRB' register values.  This will be addressed by a later patch.

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>