net/mlx4_en: Fix transmit timeout when driver restarts port

Under heavy CPU load, changing, ring size/mtu/etc. could result in transmit
timeout, since stop-start port might take more than 10 seconds.
Calling netif_detach_device to prevent tx queue transmit timeout.

netif_detach_device() is not called under ndo_stop, because netif_carrier_off
will prevent the timeout, and device should not be marked as not present, or
else user won't be able to start it later on.

CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>