cxgbe/t4_tom: completely avoid L2T entries during stop/suspend.

1. 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.
2. 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.
Sponsored by: Chelsio Communications

(cherry picked from commit 07f47e8850d0639d474026b203013072aeb32c81)