clk: Add comment about __of_clk_get_by_name() error values

It's not immediately obvious from the code that failure to get a
clock provider can return either -ENOENT or -EINVAL. Therefore, add
a comment to highlight this.
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Russell King <linux@armlinux.org.uk>
[sboyd@kernel.org: Fix comment to be proper C with ==]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clkdev.c

@@ -52,6 +52,12 @@ struct clk *of_clk_get(struct device_node *np, int index)
 }
 EXPORT_SYMBOL(of_clk_get);
 
+/*
+ * Beware the return values when np is valid, but no clock provider is found.
+ * If name == NULL, the function returns -ENOENT.
+ * If name != NULL, the function returns -EINVAL. This is because __of_clk_get()
+ * is called even if of_property_match_string() returns an error.
+ */
 static struct clk *__of_clk_get_by_name(struct device_node *np,
 					const char *dev_id,
 					const char *name)