staging: comedi: das08: rename the gainlist variables

`das08_pgh_gainlist[]`, `das08_pgl_gainlist[]`, and
`das08_pgm_gainlist[]` hold the gain codes indexed by range index for
various boards that support programmable gain. `das08_gainlist[]` is a
look-up table to find the appropriate gain list for a board.  These are
all associated with the analog input Comedi subdevice.  Rename the
variables to reflect that.

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>