[SCSI] fcoe: Fix getting san mac for VLAN interface

Make sure we are get the SAN MAC address from the real netdev if the input
netdev is a VLAN device.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>