staging: comedi: s526: add macros for counter control reg values

The driver writes a couple of literal values to the counter
control/status register, 0x8000 to reset the counter, and 0x4000 to load
the counter from preload register 0.  Add a bunch of macros to define
these values and other values for the register, based on the Sensoray
526 manual.

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>