usb: xhci: Do not panic on event timeouts

Now that we always check the return value, just return NULL on timeouts.
We can still log the error since this is a problem, but it's not reason
to panic.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index a969eaf..ae0ab57 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -494,8 +494,9 @@
 	if (expected == TRB_TRANSFER)
 		return NULL;
 
-	printf("XHCI timeout on event type %d... cannot recover.\n", expected);
-	BUG();
+	printf("XHCI timeout on event type %d...\n", expected);
+
+	return NULL;
 }
 
 /*