cxgbe/t4_tom: Do not set unresolved entries to STALE in t4_l2t_update.

An L2 entry in the driver's hash was marked STALE unconditionally if it
changed in the kernel when its driver refcount is 0. Fix the driver to
do this for VALID entries only.

Sponsored by: Chelsio Communications

(cherry picked from commit 3883300afe0bff5c5658274c4d8cfe773d08343e)