staging: comedi: dt3000: tidy up the command mailbox defines

For aesthetics, rename these defines so they are associated with
the register. Move the defines closer to the register define.

Define some macros for the "completion" and "cmd" bits.

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>