cxgbe(4): Do not request an FEC that is invalid for the requested speed.
This eliminates error messages like this from the driver when running at
50Gbps with 100G cables:
[3726] cc0: l1cfg failed: 71
[4407] cc0: l1cfg failed: 71
Note that link comes up anyway with or without this change.
Reported by: Suhas Lokesha @ Chelsio
MFC after: 1 week
Sponsored by: Chelsio Communications