vlan: Calling vlan_hwaccel_do_receive() is always valid.

It is now acceptable to receive vlan tagged packets at any time,
even if CONFIG_VLAN_8021Q is not set.  This means that calling
vlan_hwaccel_do_receive() should not result in BUG() but rather just
behave as if there were no vlan devices configured.

Reported-by: Vladislav Zolotarov <vladz@broadcom.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>