net/intel-em-kmod: unbreak for FreeBSD 15+

FreeBSD 15 introduced some incompatible KAPI changes.
Apply some patches to this code dated back 2019.
This fixes build, so PORTREVISION not changed.

The change is run-tested using 15.0-RELEASE and 82574L-based
network adapters. Compile-tested only for recent 16.0-CURRENT.