media: rc: clean the freed urb pointer to avoid double free

After freed rx_urb, we should set the second interface urb to NULL,
otherwise a double free would happen when the driver is removed
from the first interface.

Signed-off-by: Nil Yi <teroincn@163.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>