drm/omap: dsi: fix unsigned expression compared with zero

r is "u32" always >= 0,mipi_dsi_create_packet may return little than zero.
so r < 0 condition is never accessible.

Fixes coccicheck warnings:
./drivers/gpu/drm/omapdrm/dss/dsi.c:2155:5-6:
WARNING: Unsigned expression compared with zero: r < 0
Signed-off-by: Junlin Yang <yangjunlin@yulong.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210312071445.1721-1-angkery@163.com

drivers/gpu/drm/omapdrm/dss/dsi.c

@@ -2149,11 +2149,12 @@ static int dsi_vc_send_short(struct dsi_data *dsi, int vc,
 			     const struct mipi_dsi_msg *msg)
 {
 	struct mipi_dsi_packet pkt;
+	int ret;
 	u32 r;
 
-	r = mipi_dsi_create_packet(&pkt, msg);
-	if (r < 0)
-		return r;
+	ret = mipi_dsi_create_packet(&pkt, msg);
+	if (ret < 0)
+		return ret;
 
 	WARN_ON(!dsi_bus_is_locked(dsi));