staging: comedi: das16m1: tidy up interrupt control register defines

Rename these defines and use the BIT macro to define the bits. Define
some macros for the multi-bit fields in the register.

Rename the private data 'control_state' member to better match the
register name.

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>