sfc: re-factor efx_ef10_filter_sync_rx_mode()

This change is only re-factoring; there are no changes to functionality
 except for a slight elaboration of an error message (on mismatch filter
 insertion failure).

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