V4L/DVB (6810): media: video: usbvision: add mutex_unlock() to error paths

There are a few error paths which don't unlock the usbvision->lock.

So I've added mutex_unlock() calls to fix those paths.

Signed-off-by: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>