rpmsg: fix dependency on initialization order

When rpmsg drivers are built into the kernel, they must not initialize
before the rpmsg bus does, otherwise they'd trigger a BUG() in
drivers/base/driver.c line 169 (driver_register()).

To fix that, and to stop depending on arbitrary linkage ordering of
those built-in rpmsg drivers, we make the rpmsg bus initialize at
subsys_initcall.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Federico Fuga <fuga@studiofuga.com>
[ohad: rewrite the commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>

drivers/rpmsg/virtio_rpmsg_bus.c

@@ -1085,7 +1085,7 @@ static int __init rpmsg_init(void)
 
 	return ret;
 }
-module_init(rpmsg_init);
+subsys_initcall(rpmsg_init);
 
 static void __exit rpmsg_fini(void)
 {