[media] tm6000: Fix mutex unbalance

Hi

Fix never unlocked mutex. No more crash after removing USB stick.

With my best regards, Dmitry.

Signed-off-by: Beholder Intl. Ltd. Dmitry Belimov <d.belimov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>