cxgbe(4): Update firmwares to 1.26.4.0

(Rest is from the README that came with the firmware)

Version : 1.26.4.0
Date : 12/02/2021

Fixes

BASE:

• Fixed error on setting 25G speed on 100G copper with multiple FEC set in firmware commands.
• Handle link of unknown optics modules by enabling module tx unconditionally.
• Fixed link not coming up for 25G CRS phys. Firmware incorrectly tried to bring up the link in RS-FEC but as per IEEE spec, it must be BASER FEC.
• Fixed an issue where firmware doesn't automatically retry next FEC if driver asks to bring up the link using RS-FEC and link doesn't come up.

Obtained from: Chelsio Communications
Sponsored by: Chelsio Communications

(cherry picked from commit 357ba2cf17bf2a201ada5c54822e4f92ff2df296)