cxgbe(4): T7 LB mode support

This is the new mode where multiple internal channels are used to
service 1 port. LB = Load Balancing.

• Ask for tx_tpchmap from the firmware and update sc->chan_map.
• Statistics for a port should include all its channels.

Sponsored by: Chelsio Communications

(cherry picked from commit a16423b7a48c61370d4eeb542e3ca6c1b4c9579a)