staging: comedi: s626: add missing bits for 'CRB' register

There are some bits in the 'CRB' register not defined in "s626.h".
Three of these are read-only bits that overlay the write-only interrupt
control bits.  Another missing bit controls whether counter 'B' is
cleared when counter 'A' overflows.  Add the missing bit definitions for
completeness.

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>