That is already in master: https://github.com/pfsense/FreeBSD-src/commits/devel-12/sys/dev/cxgbe
So should be in 2.6 snapshots. Can you test one? Other things may be broken there....
Unfortunately I only have one production system with that card that I would reluctant to upgrade to the unstable development branch. Do I have any other options bring this fix into my current 2.5.0 environment? Thanks again.