sfc: Add support for busy polling

This patch adds the sfc driver code for implementing busy polling.
It adds ndo_busy_poll method and locking between it and napi poll.
It also adds each napi to the napi_hash right after netif_napi_add().

Uses efx_start_eventq and efx_stop_eventq in the self tests.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>