sfc: lock TX queues when calling netif_device_detach()

The assertion of netif_device_present() at the top of
efx_hard_start_xmit() may fail if we don't do this.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/selftest.c b/drivers/net/ethernet/sfc/selftest.c
index f6651d3..2069f51 100644
--- a/drivers/net/ethernet/sfc/selftest.c
+++ b/drivers/net/ethernet/sfc/selftest.c
@@ -722,7 +722,7 @@
 	/* Detach the device so the kernel doesn't transmit during the
 	 * loopback test and the watchdog timeout doesn't fire.
 	 */
-	netif_device_detach(efx->net_dev);
+	efx_device_detach_sync(efx);
 
 	if (efx->type->test_chip) {
 		rc_reset = efx->type->test_chip(efx, tests);