media: ov7251: fix mutex lock unbalance

As reported by smatch/sparse:

	drivers/media/i2c/ov7251.c:1381 ov7251_s_stream() warn: inconsistent returns '&ov7251->lock'.
	  Locked on  : 1381
	  Unlocked on: 1377

There's a lock unbalance at this routine, as it keeps the lock on
certain errors. Fix it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/i2c/ov7251.c

@@ -1378,6 +1378,7 @@ static int ov7251_s_stream(struct v4l2_subdev *subdev, int enable)
 
 err_power_down:
 	pm_runtime_put_noidle(ov7251->dev);
+	mutex_unlock(&ov7251->lock);
 	return ret;
 }