cxgb4: do not call napi_hash_del()

Calling napi_hash_del() before netif_napi_del() is dangerous
if a synchronize_rcu() is not enforced before NAPI struct freeing.

Lets leave this detail to core networking stack and feel
more comfortable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Hariprasad S <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>