axge: Skip dummy packet headers

Newer versions of the AX88179 interweave dummies alongside valid
packet headers in bulk IN transfer data. This was probably done for
backward compatibility with existing drivers.

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

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

Reviewed by: emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/842

(cherry picked from commit 70fbcd451b68b7f6038d8a602cd8d5e1bb890f1d)