ixl: Permit 802.1ad frames to pass though the chip

This patch is a quick hack to change the internal Ethertype used
within the chip. All frames with this type are dropped silently.
This patch allows you to overwrite the factory default 0x88a8, which
is used by IEEE 802.1ad VLAN stacking.

Reviewed by: kp, philip, brueffer
Approved by: kp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24179