staging: iio: accel: fix error check

sca3000_read_ctrl_reg() returns a negative number on failure, check for
this instead of zero.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/accel/sca3000_core.c

@@ -594,7 +594,7 @@ static ssize_t sca3000_read_frequency(struct device *dev,
 		goto error_ret_mut;
 	ret = sca3000_read_ctrl_reg(st, SCA3000_REG_CTRL_SEL_OUT_CTRL);
 	mutex_unlock(&st->lock);
-	if (ret)
+	if (ret < 0)
 		goto error_ret;
 	val = ret;
 	if (base_freq > 0)