ethernet: Set maximum Ethernet header length based on the capability IFCAP_VLAN_MTU

Lots of Ethernet drivers fix the header length after ether_ifattach().
Well since the net stack can conclude it based on the capability
IFCAP_VLAN_MTU, let the net stack do it rather than individual drivers.

Reviewed by: markj, glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50846

(cherry picked from commit 6ce8fd1978a9526ff0dc02ed98ef797f9154ec1f)