cxgbe(4): Consider all supported speeds when building the ifmedia list
for a port. Fix other related issues while here:

• Require port lock for access to link_config.
• Allow 100Mbps operation by tracking the speed in Mbps. Yes, really.
• New port flag to indicate that the media list is immutable. It will be used in future refinements.

This also fixes a bug where the driver reports incorrect media with
recent firmwares.

MFC after: 2 days
Sponsored by: Chelsio Communications