ixgbe: workaround errata about UDP frames with zero checksum

Intel 82599 has errata related to IPv4 UDP frames with zero checksum.
It reports such datagrams with L4 integrity errors in IXGBE_XEC
register. And after afb1aa4e6df2 commit such errors are reported
via IFCOUNTER_IERRORS. This confuses users, since actually all frames
are handled correctly by the system.
To workaround the problem, let's ignore the XEC register value for
82599 cards for now.

PR: 266048
Discussed with: erj
MFC after: 1 week
Sponsored by: Yandex LLC