media: cec-adap.c: drop activate_cnt, use state info instead

Using an activation counter to decide when the enable or disable the
cec adapter is not the best approach and can lead to race conditions.

Change this to determining the current status of the adapter, and
enable or disable the adapter accordingly.

It now only needs to be called whenever there is a chance that the
state changes, and it can handle enabling/disabling monitoring as
well if needed.

This simplifies the code and it should be a more robust approach as well.

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