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>