sfc: Distinguish queue lookup from test for queue existence

efx_channel_get_{rx,tx}_queue() currently return NULL if the channel
isn't used for traffic in that direction.  In most cases this is a
bug, but some callers rely on it as an existence test.

Add existence test functions efx_channel_has_{rx_queue,tx_queues}()
and use them as appropriate.

Change efx_channel_get_{rx,tx}_queue() to assert that the requested
queue exists.

Remove now-redundant initialisation from efx_set_channels().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/sfc/efx.c b/drivers/net/sfc/efx.c
index c559bc3..6189d30 100644
--- a/drivers/net/sfc/efx.c
+++ b/drivers/net/sfc/efx.c
@@ -1271,21 +1271,8 @@
 
 static void efx_set_channels(struct efx_nic *efx)
 {
-	struct efx_channel *channel;
-	struct efx_tx_queue *tx_queue;
-
 	efx->tx_channel_offset =
 		separate_tx_channels ? efx->n_channels - efx->n_tx_channels : 0;
-
-	/* Channel pointers were set in efx_init_struct() but we now
-	 * need to clear them for TX queues in any RX-only channels. */
-	efx_for_each_channel(channel, efx) {
-		if (channel->channel - efx->tx_channel_offset >=
-		    efx->n_tx_channels) {
-			efx_for_each_channel_tx_queue(tx_queue, channel)
-				tx_queue->channel = NULL;
-		}
-	}
 }
 
 static int efx_probe_nic(struct efx_nic *efx)
@@ -1531,9 +1518,9 @@
 	efx->irq_rx_adaptive = rx_adaptive;
 	efx->irq_rx_moderation = rx_ticks;
 	efx_for_each_channel(channel, efx) {
-		if (efx_channel_get_rx_queue(channel))
+		if (efx_channel_has_rx_queue(channel))
 			channel->irq_moderation = rx_ticks;
-		else if (efx_channel_get_tx_queue(channel, 0))
+		else if (efx_channel_has_tx_queues(channel))
 			channel->irq_moderation = tx_ticks;
 	}
 }