cxgbe(4): internal knob for flexible control over FEC selection.

Recent firmwares have support for autonomous FEC selection and a "force"
knob to let the driver control this behavior (or not) in a fine grained
manner. This change adds a driver knob so that all the different ways of
configuring the link FEC can be exercised. Note that this controls the
internal driver/firmware interaction for link configuration and is not
meant for general use.

MFC after: 1 week
Sponsored by: Chelsio Communications