V4L/DVB (8544): gspca: probe/open race.

The device is flagged present after it is registered. During that window calls
to open() that should work fail with -ENODEV. Reversing the order fixes
the race.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>