net: export dev_set_threaded symbol

For wireless devices (e.g. mt76 driver) multiple net_devices belongs to
the same wireless phy and the napi object is registered in a dummy
netdevice related to the wireless phy.
Export dev_set_threaded in order to be reused in device drivers enabling
threaded NAPI.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>