drivers: eth: stm32: Add VLAN support

Add Virtual LAN support to stm32 Ethernet driver. Refactor the
eth_iface_init() and move device configuration settings to
eth_initialize() as the eth_iface_init() is called multiple
times (once / configured VLAN).

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>