sfc: fix W=1 warnings in efx_farch_handle_rx_not_ok

Some of these RX-event flags aren't used at all, so remove them.
Others are used only #ifdef DEBUG to log a message; suppress the
 unused-var warnings #ifndef DEBUG with a void cast.

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