media: au0828: fix unbalanced lock/unlock in au0828_usb_probe

Call mutex_unlock and free dev on failure.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>