dpaa2: make software VLANs usable on dpni

dpni announces IFCAP_VLAN_MTU but internally does not increase the
maximum frame length. Createing a vlan interface on top of a dpni
interface will result in full-sized frames not passing.
Extend the maximum frame length by ETHER_VLAN_ENCAP_LEN to allow at
least for one layer of (software) vlans for now

MFC after: 3 days
GH-Issue: https://github.com/mcusim/freebsd-src/issues/22
Reviewed by: dsl
Differential Revision: https://reviews.freebsd.org/D42645