hiddev: Return EPOLLOUT from hiddev_poll

Always return EPOLLOUT from hiddev_poll when a device is connected.
This is safe since hiddev_write always fails and improves compatibility
with tools like socat.

Signed-off-by: Fabian Henneke <fabian@henneke.me>
In-reply-to: <nycvar.YFH.7.76.1907171333160.5899@cbobk.fhfr.pm>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>