clk: ti: fix ti_clk_get_reg_addr error handling

There is a case where NULL can be a valid return value for
ti_clk_get_reg_addr, specifically the case where both the provider index
and register offsets are zero. In this case, the current error checking
against a NULL pointer will fail. Thus, change the API to return a
ERR_PTR value in an error case, and change all the users of this API to
check against IS_ERR instead.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Michael Turquette <mturquette@linaro.org>

drivers/clk/ti/autoidle.c

@@ -119,7 +119,7 @@ int __init of_ti_clk_autoidle_setup(struct device_node *node)
 	clk->name = node->name;
 	clk->reg = ti_clk_get_reg_addr(node, 0);
 
-	if (!clk->reg) {
+	if (IS_ERR(clk->reg)) {
 		kfree(clk);
 		return -EINVAL;
 	}