staging: comedi: dt9812: default the analog input gain to 1

The 'default' case should never happen. In case it does, default the
gain to '1'.

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