qlcnic: guard __vlan_find_dev_deep() by rcu_read_lock

rcu_read_lock was missing here

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Sony Chacko <sony.chacko@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>