staging: comedi: pcl812: factor analog input range selection out of (*attach)

The analog input subdevice range is setup in this driver based on a config
option passed by the user. Factor the code that sets the range_table out
of pcl812_attach() to clarify the (*attach).

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