drm/dsi: Fix improper use of mipi_dsi_device_transfer() return value

The function mipi_dsi_device_transfer() returns the number of transmitted
or received bytes on success or a negative error code on failure.

The functions mipi_dsi_shutdown_peripheral(), mipi_dsi_turn_on_peripheral() &
mipi_dsi_set_maximum_return_packet_size() use improperly this returned
value in case of success: 0 should be returned instead of the number of
transmitted bytes.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180112144847.18810-1-philippe.cornu@st.com

drivers/gpu/drm/drm_mipi_dsi.c

@@ -498,8 +498,9 @@ int mipi_dsi_shutdown_peripheral(struct mipi_dsi_device *dsi)
 		.tx_buf = (u8 [2]) { 0, 0 },
 		.tx_len = 2,
 	};
+	int ret = mipi_dsi_device_transfer(dsi, &msg);
 
-	return mipi_dsi_device_transfer(dsi, &msg);
+	return (ret < 0) ? ret : 0;
 }
 EXPORT_SYMBOL(mipi_dsi_shutdown_peripheral);
 
@@ -517,8 +518,9 @@ int mipi_dsi_turn_on_peripheral(struct mipi_dsi_device *dsi)
 		.tx_buf = (u8 [2]) { 0, 0 },
 		.tx_len = 2,
 	};
+	int ret = mipi_dsi_device_transfer(dsi, &msg);
 
-	return mipi_dsi_device_transfer(dsi, &msg);
+	return (ret < 0) ? ret : 0;
 }
 EXPORT_SYMBOL(mipi_dsi_turn_on_peripheral);
 
@@ -541,8 +543,9 @@ int mipi_dsi_set_maximum_return_packet_size(struct mipi_dsi_device *dsi,
 		.tx_len = sizeof(tx),
 		.tx_buf = tx,
 	};
+	int ret = mipi_dsi_device_transfer(dsi, &msg);
 
-	return mipi_dsi_device_transfer(dsi, &msg);
+	return (ret < 0) ? ret : 0;
 }
 EXPORT_SYMBOL(mipi_dsi_set_maximum_return_packet_size);