dm: core: Don't allow uclass use before ready

At present it is possible to call uclass_get() before driver model is
inited. In fact this happens on x86 boards which use Intel FSPv1, since
mrccache_get_region() tries to get the SPI flash device very early
during init.

This has always been undefined behaviour. Previously it generally worked,
i.e. returned an error code without crashing, because gd->uclass_root_s
is zeroed and the uclass can be added despite driver model not being
ready, due to the way lists are implemented. With the change to use a
gd->uclass_root pointer, this no-longer works. For example, it causes a
hang on minnowmax.

Fix this by adding a check that driver model is ready when uclass_get() is
called. This function is called in the process of locating any device, so
it is a good place to add the check.

This fixes booting on minnowmax.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 8a715530bb ("dm: core: Allow the uclass list to move")

drivers/core/uclass.c

@@ -146,6 +146,9 @@ int uclass_get(enum uclass_id id, struct uclass **ucp)
 {
 	struct uclass *uc;
 
+	/* Immediately fail if driver model is not set up */
+	if (!gd->uclass_root)
+		return -EDEADLK;
 	*ucp = NULL;
 	uc = uclass_find(id);
 	if (!uc) {