ipq40xx: essedma: enable VLAN tag offload for single-port

ipq40xx: essedma: enable VLAN tag offload for single-port

Enable the VLAN tag offloading mechanism for RGMII single-port devices.
This allows those devices to use 802.1Q VLANs on the ethernet port.

Previously, RX frames were double tagged, as the RX TAG removal flag was
not enabled and an additional 802.1Q header was inserted elsewhere in
the code.

On the TX side, tagging was completely not present for single-port
devices. Enable tagging if an 802.1Q frame should be transmitted and
disable the default tagging mechanism for single-port devices.

Tested on Aruba AP-303

Signed-off-by: David Bauer <mail@david-bauer.net>