em28xx-i2c: rt_mutex_trylock() returns zero on failure

BugLink: http://bugs.launchpad.net/bugs/1631468

commit e44c153b upstream.

The code is checking for negative returns but it should be checking for
zero.

Fixes: aab3125c ('[media] em28xx: add support for registering multiple i2c buses')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/media/usb/em28xx/em28xx-i2c.c

@@ -507,9 +507,8 @@ static int em28xx_i2c_xfer(struct i2c_adapter *i2c_adap,
 	if (dev->disconnected)
 		return -ENODEV;
 
-	rc = rt_mutex_trylock(&dev->i2c_bus_lock);
-	if (rc < 0)
-		return rc;
+	if (!rt_mutex_trylock(&dev->i2c_bus_lock))
+		return -EAGAIN;
 
 	/* Switch I2C bus if needed */
 	if (bus != dev->cur_i2c_bus &&