media: cec-core: first mark device unregistered, then wake up fhs

If a CEC device node is unregistered, then it should be marked as
unregistered before waking up any filehandles that are waiting for
an event.

This ensures that there is no race condition where an application can
call CEC_DQEVENT and have the ioctl return 0 instead of ENODEV.

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