net: fec: fec_enet_rx_queue(): replace manual VLAN header calculation with skb_vlan_e...

net: fec: fec_enet_rx_queue(): replace manual VLAN header calculation with skb_vlan_eth_hdr()

For better readability and maintainability, use the provided helper function
skb_vlan_eth_hdr() to replace manual the VLAN header calculation, and change
the type of vlan_header to struct vlan_ethhdr to take into account that the
Ethernet header plus VLAN header is returned.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://patch.msgid.link/20250618-fec-cleanups-v4-8-c16f9a1af124@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>