sfc: Rework efx_set_multicast_hash()

When !port_enabled, defer the write to reconfigure_mac_wrapper.

Whilst here, simplify the logic now that efx_start_port() always calls
efx_reconfigure_port().

From: Steve Hodgson <shodgson@solarflare.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>