[media] tm6000: Add locking for USB transfers

This commit introduces the usb_lock mutex to ensure that a USB request
always gets the proper response. While this is currently not really
necessary it will become important as there are more users.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>