staging: comedi: das08: replace DAS08AO_GAIN_CONTROL/STATUS

The `DAS08AO_GAIN_CONTROL` and `DAS08AO_GAIN_STATUS` macros hold the
offset to the "programmable gain" register on "PGL", "PGM", "PGH",
"AOL", "AOM" and "AOH" boards.  Writing a code to this register sets the
gain for the current analog input channel (selected in the main control
register).  The written value can be read back in bits 3..0 of the
register.  Other bits of the register are read-only and not used by the
driver.  Rename `DAS08AO_GAIN_CONTROL` to `DAS08_GAIN_REG` and add a
comment.  Remove `DAS08AO_GAIN_STATUS` as the driver does not use it and
the read-only parts of the register are documented in the comment.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>