cxgbe(4): Do not configure traffic classes automatically on attach.

The driver used to configure all available classes with some default
parameters on attach and the rest of t4_sched.c was written with the
assumption that all traffic classes are always valid in the hardware.
But this resulted in a lot of informational messages being logged in the
firmware's circular log, crowding out other more useful messages.

This change leaves the tx scheduler alone during attach to reduce the
spam in the devlog. The state of every class is now tracked separately
from its flags and there is support for an 'uninitialized' state.

Sponsored by: Chelsio Communications

(cherry picked from commit ec8004dd419d8c8acfc9025dd050f141c949d53a)