cxgbe(4): Update firmwares to 1.26.2.0.

The firmwares and the following changelog are from the "Chelsio Unified
Wire v3.15.0.0 for Linux."

Version : 1.26.2.0

Date : 09/24/2021

FIXES

BASE:

• Added support for SFP+ RJ45 (0x1C).
• Fixing backward compatibility issue with older drivers when multiple speeds are passed to firmware.

OFLD:

• Do not touch tp_plen_max if driver is supplying tp_plen_max. This fixes a connection reset issue in iscsi.

ENHANCEMENTS

BASE:

• Firmware header modified to add firmware binary signature.

Sponsored by: Chelsio Communications

(cherry picked from commit 45d6fbaec23eee457197a14517e715c947114d99)