V4L/DVB: uvcvideo: Use POLLOUT and POLLWRNORM for output devices

The V4L2 specification requires drivers to use the write events in the
file operations poll handler for output devices. The uvcvideo driver
erroneously used read events for all devices. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>