the firmware (instead of just the main firmware version) when evaluating
firmware compatibility. Document the new "hw.cxgbe.fw_install" knob
being introduced here.
This should fix kern/173584 too. Setting hw.cxgbe.fw_install=2 will
mostly do what was requested in the PR but it's a bit more intelligent
in that it won't reinstall the same firmware repeatedly if the knob is
left set.
PR: kern/173584
MFC after: 5 days