staging: comedi: pcm3724: tidy up register map defines

For aesthetics, rename the register map defines so they have namespace
associated with the driver and use the BIT macro to define the bits.

Add the missing defines for the registers and use them to remove the
magic values from the driver.

Add a comment about the extra registers to explain why this driver
doesn't just use the standard 8255 driver instead.

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>