cxgbe(4): Handle FORCE_FEC in pcaps correctly.

The firmware doesn't report FORCE_FEC in pcaps if the transceiver
plugged in at that time does not support a speed that may use FEC. It
is incorrect for the driver to assume that the FORCE_FEC value it read
during attach (in init_link_config) is permanent. Instead, it should
check pcaps just before issuing the L1CFG command.

MFC after: 1 week
Sponsored by: Chelsio Communications