staging: comedi: das08: improve test for programmable gain

`das08_ai_rinsn()` handles Comedi `INSN_READ` instructions for the AI
subdevice.  This programs the gain for the analog input channel if the
board has support for that, and acquires data from the channel.  If
programmable gain is supported, the gain code is read from the array
pointed to by `devpriv->pg_gainlist` indexed by the range index.  The
function assumes that programmable gain is supported if the AI
subdevice's range table supports more than one range.  Replace that with
a more direct test for `devpriv->pg_gainlist` being non-NULL, as it is
only initialized to a non-NULL pointer for boards that support
programmable gain.

This will also allow range tables to be included for convenience for
those boards that support multiple ranges by DIP switches.  Those boards
are currently initialized to use a single "unknown" range.

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>