sfc: remove tx and MCDI handling from NAPI budget consideration

The NAPI budget is only for RX processing work, not other work such as
 TX or MCDI completion handling.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>