iio:adc:vf610 ensure correct handing of interruption of wait for completion

Issue highlighted by smatch warning:

CHECK   drivers/iio/adc/vf610_adc.c
drivers/iio/adc/vf610_adc.c:466 vf610_read_raw() warn: unsigned 'ret' is never less than zero.

As wait_for_completion_interruptible_timeout can return -ERESTARTSTSYS, if
interrupted, ret must be signed.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Fugang Duan <B38611@freescale.com>

drivers/iio/adc/vf610_adc.c

@@ -447,7 +447,7 @@ static int vf610_read_raw(struct iio_dev *indio_dev,
 {
 	struct vf610_adc *info = iio_priv(indio_dev);
 	unsigned int hc_cfg;
-	unsigned long ret;
+	long ret;
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW: