[media] cec: rename cec_devnode fhs_lock to just lock

This lock will be used to protect more than just the fhs list.
So rename it to just 'lock'.

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