staging: comedi: das08: use indexed initializer for AI range table types

The "das08" common module for DAS08 ISA, PCI, and PCMCIA drivers
includes a predefined set of AI range tables.  The static board data (of
type `struct das08_board_struct`) for a particular board contains an
index in its `ai_pg` member (of type `enum das08_lrange`) indicating
which of the predefined AI range tables to use.  The "das08" common
module looks up this index in `das08_ai_lranges[]` to get a pointer to
the predefined range table for the board.  The same index is also looked
up in `das08_gainlists[]` to get a corresponding pointer to a list of
hardware gain values for each range supported by the board (NULL for
boards without programmable gain).

To make this clearer, used indexed initializers for `das08_ai_lranges[]`
and `das08_gainlists[]`, using the enumerated constants from `enum
das08_lrange` as the indices.  Also add a short comment to the
definition of `enum das08_lrange`.

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>