Stop the URBs in em28xx_stop_streaming(), so that em28xx_irq_callback()
cannot be called after the streaming has stopped.

This should eliminate the crashes reported by Antti Palosaari and the warnings
reported by Andy Furniss.
Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>