V4L/DVB (7491): vivi: make vivi openable only once

vivi currently doesn't have the infrastructure to handle being opened more than
one time and will crash if it is.  So, make it openable only once.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>