[media] cec: improve flushing queue

When the adapter is unloaded or unconfigured, then all transmits and
pending waits should be flushed.

Move this code into its own function and improve the code that cancels
delayed work to avoid having to unlock adap->lock.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>