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>

drivers/staging/comedi/drivers/pcmad.c

@@ -88,7 +88,6 @@ static int pcmad_ai_insn_read(struct comedi_device *dev,
 			      struct comedi_subdevice *s,
 			      struct comedi_insn *insn, unsigned int *data)
 {
-	const struct pcmad_board_struct *board = comedi_board(dev);
 	struct pcmad_priv_struct *devpriv = dev->private;
 	int chan;
 	int n;
@@ -107,7 +106,7 @@ static int pcmad_ai_insn_read(struct comedi_device *dev,
 		data[n] |= (inb(dev->iobase + PCMAD_MSB) << 8);
 
 		if (devpriv->twos_comp)
-			data[n] ^= (1 << (board->n_ai_bits - 1));
+			data[n] ^= ((s->maxdata + 1) >> 1);
 	}
 
 	return n;