sfc: Call netif_napi_add() before registering our interrupt handler

If we use a shared legacy IRQ then our interrupt handler may be called
as soon as it is registered even though IRQs are disabled on the NIC.
Now that the legacy interrupt handler also checks for event delivery,
it may decide to schedule polling in this case.  Ensure that the NAPI
context is valid but disabled at this point.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>