media: v4l2-ctrls.c: fix epoll() by calling poll_wait first

The epoll function expects that whenever the poll file op is
called, the poll_wait function is also called. That didn't
always happen in v4l2_ctrl_poll(). Fix this, otherwise epoll()
would timeout when it shouldn't.

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