clk: Skip clk provider registration when np is NULL

commit 6579c8d ("clk: Mark fwnodes when their clock provider is added")
revealed that clk/bcm/clk-raspberrypi.c driver calls
devm_of_clk_add_hw_provider(), with a NULL dev->of_node, which resulted in a
NULL pointer dereference in of_clk_add_hw_provider() when calling
fwnode_dev_initialized().

Returning 0 is reducing the if conditions in driver code and is being
consistent with the CONFIG_OF=n inline stub that returns 0 when CONFIG_OF
is disabled. The downside is that drivers will maybe register clkdev lookups
when they don't need to and waste some memory.

Fixes: 6579c8d ("clk: Mark fwnodes when their clock provider is added")
Fixes: 3c9ea42 ("clk: Mark fwnodes when their clock provider is added/removed")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Saravana Kannan <saravanak@google.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210426065618.588144-1-tudor.ambarus@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/clk/clk.c

@@ -4540,6 +4540,9 @@ int of_clk_add_provider(struct device_node *np,
 	struct of_clk_provider *cp;
 	int ret;
 
+	if (!np)
+		return 0;
+
 	cp = kzalloc(sizeof(*cp), GFP_KERNEL);
 	if (!cp)
 		return -ENOMEM;
@@ -4579,6 +4582,9 @@ int of_clk_add_hw_provider(struct device_node *np,
 	struct of_clk_provider *cp;
 	int ret;
 
+	if (!np)
+		return 0;
+
 	cp = kzalloc(sizeof(*cp), GFP_KERNEL);
 	if (!cp)
 		return -ENOMEM;
@@ -4676,6 +4682,9 @@ void of_clk_del_provider(struct device_node *np)
 {
 	struct of_clk_provider *cp;
 
+	if (!np)
+		return;
+
 	mutex_lock(&of_clk_mutex);
 	list_for_each_entry(cp, &of_clk_providers, link) {
 		if (cp->node == np) {