drm/bridge: waveshare-dsi: Fix bailout for devm_drm_bridge_alloc()

devm_drm_bridge_alloc() returns ERR_PTR on failure instead of a
NULL pointer, so use IS_ERR() to check the returned pointer and
turn proper error code on failure by using PTR_ERR().

Fixes: dbdea37add13 ("drm: bridge: Add waveshare DSI2DPI unit driver")
Signed-off-by: Liu Ying <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/bridge/waveshare-dsi.c

···
 	int ret;
 
 	ws = devm_drm_bridge_alloc(dev, struct ws_bridge, bridge, &ws_bridge_bridge_funcs);
-	if (!ws)
-		return -ENOMEM;
+	if (IS_ERR(ws))
+		return PTR_ERR(ws);
 
 	ws->dev = dev;