cxgbe(4): Better defaults for all cl-rl rate limiters.

Start in "class" instead of "flow" mode. This eliminates the need to
specify an MTU, which is not available that early anyway. It also
allows the user to manually configure ch-rl rate limiting after attach.
This used to fail because ch-rl isn't supported if cl-rl "flow" mode is
configured.

Set all traffic classes to 1Gbps during initialization. The goal is to
start off with _any_ valid configuration and 1Gbps works even for
gigabit cards.

Sponsored by: Chelsio Communications