iio:cdc:ad7150: Fix use of uninitialized ret

This doesn't appear to generate a warning on all versions of GCC, but
0-day reported it and the report looks valid.

Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/cdc/ad7150.c

···
 				     enum iio_event_direction dir, int state)
 {
 	struct ad7150_chip_info *chip = iio_priv(indio_dev);
-	int ret;
+	int ret = 0;
 
 	/*
 	 * There is only a single shared control and no on chip