myri10ge: fix rmmod crash

Rmmod myri10ge crash at free_netdev() -> netif_napi_del(), because napi
structures are already deallocated. To fix call netif_napi_del() before
kfree() at myri10ge_free_slices().

Cc: stable@kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>