staging: comedi: dt2811: tidy up A/D Gain/Channel register defines

Cleanup the defines for this register and its bits and remove the
redundant information in the comment.

Make the (*insn_read) use the range to set the gain bits correctly.
Currently the gain is always set to 1.

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>