cxgbe(4): Allow tracing filters on loopback ports.

Each physical port has an associated loopback tx channel and anything
transmitted over that channel by the driver is looped back internally by
the hardware as if received on that physical port. This change allows
tracing filters to be installed in this loopback path.

Sponsored by: Chelsio Communications

(cherry picked from commit 9f354cd3d0170f0deba2a548e7894d48aa67ee23)