cxgbe(4): separate sysctls for user-requested and in-use FEC.

Recent firmwares have more leeway in FEC selection and there is a need
to track the FECs requested by the driver separately from the FEC in use
on the link. The existing dev.<port>.<inst>.fec sysctl can read both but
its behavior depends on the link state and it is sometimes hard to find
out what was requested when the link is up.

Split the fec sysctl into two (requested_fec and link_fec) to get access
to both pieces of information regardless of the link state.

Sponsored by: Chelsio Communications

(cherry picked from commit f6a2e1100fa09742aa29d8e51fd6181fb5c171bb)