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>
diff --git a/drivers/clk/ti/divider.c b/drivers/clk/ti/divider.c
index 6211893..ff5f117 100644
--- a/drivers/clk/ti/divider.c
+++ b/drivers/clk/ti/divider.c
@@ -530,8 +530,8 @@
 	u32 val;
 
 	*reg = ti_clk_get_reg_addr(node, 0);
-	if (!*reg)
-		return -EINVAL;
+	if (IS_ERR(*reg))
+		return PTR_ERR(*reg);
 
 	if (!of_property_read_u32(node, "ti,bit-shift", &val))
 		*shift = val;