cxgbe(4): Update firmwares to 1.26.6.0.

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CHANGES
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Version : 1.26.6.0

Date : 01/03/2022

Fixes

BASE:

• Fixed one module eeprom read failure.
• Fixed an issue with speed selection when 40G and 25G are advertised and supported.
• Fixed a random traffic hang when T5 receives invalid ets BW in dcbx messages from a switch.
• Fixed very long link up time with few switches.

Obtained from: Chelsio Communications
MFC after: 2 weeks
Sponsored by: Chelsio Communications