cxgbe(4): Allow t4_tom to be unloaded safely.

• Disable IFCAP_TOE automatically on all ifnets on all adapters during unload. This is user-friendly and avoids panics due to stale ifnet state after t4_tom is unloaded.
• Do not allow unload if tids are in use by the TOE on any adapter.

Reported by: Bimal Abraham @ Chelsio
MFC after: 1 week
Sponsored by: Chelsio Communications