net: vlan, qlcnic: make vlan_find_dev private

there is only one user of vlan_find_dev outside of the actual vlan code:
qlcnic uses it to iterate over some VLANs it knows.

let's just make vlan_find_dev private to the VLAN code and have the
iteration in qlcnic be a bit more direct. (a few rcu dereferences less
too)

Signed-off-by: David Lamparter <equinox@diac24.net>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Amit Kumar Salecha <amit.salecha@qlogic.com>
Cc: Anirban Chakraborty <anirban.chakraborty@qlogic.com>
Cc: linux-driver@qlogic.com
Signed-off-by: David S. Miller <davem@davemloft.net>