staging: comedi: das08: rename ai range tables

The `range_das08_pgl`, `range_das08_pgm`, and `range_das08_pgh`
variables define the analog input ranges for the "PGL", "PGM", and "PGH"
board variants, and are also used for the "AOL", "AOM", and "AOH" board
variants.  Rename them to use the `das08_` prefix for consistency.

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>