[media] cx88: fix locking of sub-driver operations
The BKL conversion of this driver seems to have gone wrong.
Loading the cx88-blackbird driver deadlocks.
The cause: mpeg_ops::open in the cx2388x blackbird driver acquires the
device lock and calls the sub-driver's request_acquire, which tries to
acquire the lock again. Fix it by clarifying the semantics of
request_acquire, request_release, advise_acquire, and advise_release:
now all will rely on the caller to acquire the device lock.
Based on work by Ben Hutchings <ben@decadent.org.uk>.