cxgbe(4): Update firmwares to 1.26.4.0

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
MFC after:	1 month
Sponsored by:	Chelsio Communications