media: seco-cec: decrement HDMI device refcount

The SECO CEC driver never decremented the HDMI device refcount.
CEC drivers only need the HDMI device pointer as a key in the
notifier list, it never accesses the device, so there is no
need to keep a reference.

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