media: cec: add EPOLLPRI in poll() when dev is unregistered

If the CEC device was unregistered, then add EPOLLPRI to
the poll() mask. Otherwise a select() that only waits for
exceptions will not wake up. A select() that waits for
read and/or write events *will* wake up on an EPOLLERR, but
not (for some reason) if it just waits for exceptions.

Strangly the epoll functionality will wakeup on EPOLLERR if
you just wait for an exception, so in this respect select()
and epoll differ.

In the end it doesn't really matter, what matters is that
polling file handles are woken up on device unregistration.

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