V4L/DVB (13398): firedtv: reform lock transaction backend call

Preparation for the port of firedtv to the firewire-core kernel API:
The fdtv->backend->lock() hook and thus the CMP code is slightly changed
to better fit with the new API.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>