[media] xc2028: unlock on error in xc2028_set_config()

We have to unlock before returning -ENOMEM.

Fixes: 8dfbcc43

 ('[media] xc2028: avoid use after free')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>