staging: comedi: s626: define CRA/CRB registers based on channel number

Start cleaning up the 'struct s626_enc_info' by redefining the my_cra
and my_crb members based on the comedi channel number.

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