staging: comedi: rtd520: remove 'aiMaxGain' from boardinfo

This value is only used in the attach to determine which range
table the analog input subdevice uses. Remove this variable and
just pass the range table pointer in the boardinfo.

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>