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
Sponsored by: Chelsio Communications

(cherry picked from commit f3c2987f2f5c91f0801fa8bbf9e1bc09d91aeb46)