staging: comedi: pcmad: fix analog input data on pcmad12

According to the PCM-A/D-12/16 Operations Manual:

NOTE: On the PCM-A/D-12 the lower nibble of the hex value will always be 0.

Fix the pcmad_ai_insn_read() function to properly handle this by shifting
the data after it has been read.
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

@@ -104,6 +104,10 @@ static int pcmad_ai_insn_read(struct comedi_device *dev,
 		val = inb(dev->iobase + PCMAD_LSB) |
 		      (inb(dev->iobase + PCMAD_MSB) << 8);
 
+		/* data is shifted on the pcmad12, fix it */
+		if (s->maxdata == 0x0fff)
+			val >>= 4;
+
 		if (pcmad_range_is_bipolar(s, range)) {
 			/* munge the two's complement value */
 			val ^= ((s->maxdata + 1) >> 1);