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/interface.c

@@ -111,7 +111,7 @@ static void __init _of_ti_interface_clk_setup(struct device_node *node,
 	u32 val;
 
 	reg = ti_clk_get_reg_addr(node, 0);
-	if (!reg)
+	if (IS_ERR(reg))
 		return;
 
 	if (!of_property_read_u32(node, "ti,bit-shift", &val))