staging: comedi: plx9052.h: document the CNTRL register

Add defines for the PLX 9052 CNTRL register defines and use them
instead of the magic numbers in the me4000 driver

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>