V4L/DVB (6237): Oops in pwc v4l driver

The pwc driver is defficient in locking, which can trigger an oops
when disconnecting.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
CC: Luc Saillard <luc@saillard.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>