staging: comedi: ni_labpc: remove *_ai_gain_bits tables

The bits needed to set the analog input gain can be simply calculated
based on the 'range'.

The LabPC versions of the board do not have the '0x10' gain that the
LabPC+ board supports. By incrementing the range appropriately the
correct gain bits can still be calculated.

This allows removing the two gain tables, as well as the export, along
with the 'ai_range_code' data in the boardinfo.

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