media: pulse8-cec: locking improvements

Drop the write_lock, rename config_lock to plain lock since this
now locks access to the adapter. Use 'lock' when transmitting
a message, ensuring that nothing interferes with the transmit.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>