staging: comedi: pcmad: remove need for boardinfo in pcmad_ai_insn_read()

The const boardinfo pointer is only used in this function to work out
the value needed to convert the data to its twos complement. Use the
comedi_subdevice 'maxdata' to do this conversion and remove the 'board'
pointer.

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>