cxgbe(4): Updates to link configuration.

• Update struct link_settings and associated shared code.

• Add tunables to control FEC and autonegotiation. All ports inherit these values as their initial settings. hw.cxgbe.fec hw.cxgbe.autoneg

• Add per-port sysctls to control FEC and autonegotiation. These can be modified at any time. dev.<port>.<n>.fec dev.<port>.<n>.autoneg

MFC after: 3 days
Sponsored by: Chelsio Communications