ixgbe: cleanup ixgbe_ndo_set_vf_vlan

Clean up functionality in ixgbe_ndo_set_vf_vlan that will simplify later
patches.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>