staging: comedi: ni_mio_common: fix E series ni_ai_insn_read() data

Commit 0557344e ("staging: comedi: ni_mio_common: fix local var for
32-bit read") changed the type of local variable `d` from `unsigned
short` to `unsigned int` to fix a bug introduced in
commit 9c340ac9 ("staging: comedi: ni_stc.h: add read/write
callbacks to struct ni_private") when reading AI data for NI PCI-6110
and PCI-6111 cards.  Unfortunately, other parts of the function rely on
the variable being `unsigned short` when an offset value in local
variable `signbits` is added to `d` before writing the value to the
`data` array:

			d += signbits;
		  	data[n] = d;

The `signbits` variable will be non-zero in bipolar mode, and is used to
convert the hardware's 2's complement, 16-bit numbers to Comedi's
straight binary sample format (with 0 representing the most negative
voltage).  This breaks because `d` is now 32 bits wide instead of 16
bits wide, so after the addition of `signbits`, `data[n]` ends up being
set to values above 65536 for negative voltages.  This affects all
supported "E series" cards except PCI-6143 (and PXI-6143). Fix it by
ANDing the value written to the `data[n]` with the mask 0xffff.

Fixes: 0557344e ("staging: comedi: ni_mio_common: fix local var for 32-bit read")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org>
----
Needs backporting to stable kernels 3.18 onwards.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_mio_common.c

@@ -1875,7 +1875,7 @@ static int ni_ai_insn_read(struct comedi_device *dev,
 				return -ETIME;
 			}
 			d += signbits;
-			data[n] = d;
+			data[n] = d & 0xffff;
 		}
 	} else if (devpriv->is_6143) {
 		for (n = 0; n < insn->n; n++) {
@@ -1924,9 +1924,8 @@ static int ni_ai_insn_read(struct comedi_device *dev,
 				data[n] = dl;
 			} else {
 				d = ni_readw(dev, NI_E_AI_FIFO_DATA_REG);
-				/* subtle: needs to be short addition */
 				d += signbits;
-				data[n] = d;
+				data[n] = d & 0xffff;
 			}
 		}
 	}