cxgbe(4): Fix stats collection for ports with port_id != tx_chan

This fixes a driver panic during stats collection when a port's id does
not match its tx channel. The bug affected only the T580 card running
with a non-default VPD.

Reported by: Suhas Lokesha @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit bbab9ab5796028e6ef9d765bfd307936bde14e89)