cxgbe/t4_tom: completely avoid L2T entries during stop/suspend.
- Mark the L2T entry valid only if t4_write_l2e succeeds, which won't happen if the adapter is stopped. This prevents L2T entries from sometimes getting (re)promoted to VALID on Tx activity during stop.
- Discard a work request immediately instead of enqueueing it to the arp queue if the adapter is stopped.
Fixes: c1c524852f62 cxgbe/t4_tom: Implement uld_stop and uld_restart for ULD_TOM.
MFC after: 1 week
Sponsored by: Chelsio Communications