Add support for AX88179A. AX88179A interweave dummies alongside valid

Add support for AX88179A.  AX88179A interweave dummies alongside valid
packet headers in axen_rxeof().

However current driver records these dummy headers as dropped frames,
leading to stats misreporting one Ifail per Ipkt.

This skips those dummy headers silently, thereby not generating Ifail
for them.

From FreeBSD commit 70fbcd451b68b7f6038d8a602cd8d5e1bb890f1d

Tested by landry@ and myself.
ok claudio@, landry@