sfc: Fix loopback self-test with separate_tx_channels=1

The loopback self-test iterates over all the TX queues of channel 0,
which is not very interesting when that's an RX-only channel.
Signed-off-by: Ben Hutchings <bhutchings@solarflre.com>

drivers/net/ethernet/sfc/ethtool.c

@@ -337,7 +337,8 @@ static int efx_fill_loopback_test(struct efx_nic *efx,
 				  unsigned int test_index,
 				  struct ethtool_string *strings, u64 *data)
 {
-	struct efx_channel *channel = efx_get_channel(efx, 0);
+	struct efx_channel *channel =
+		efx_get_channel(efx, efx->tx_channel_offset);
 	struct efx_tx_queue *tx_queue;
 
 	efx_for_each_channel_tx_queue(tx_queue, channel) {

drivers/net/ethernet/sfc/selftest.c

@@ -614,7 +614,8 @@ static int efx_test_loopbacks(struct efx_nic *efx, struct efx_self_tests *tests,
 {
 	enum efx_loopback_mode mode;
 	struct efx_loopback_state *state;
-	struct efx_channel *channel = efx_get_channel(efx, 0);
+	struct efx_channel *channel =
+		efx_get_channel(efx, efx->tx_channel_offset);
 	struct efx_tx_queue *tx_queue;
 	int rc = 0;