clk: fixup default index for of_clk_get_by_name()

of_clk_get_by_name() is using -1 for __of_clk_get() index.  It will go
to of_parse_clkspec(), and be used for of_parse_phandle_with_args().
Here, if user doesn't specify clock name (= of_clk_get_by_name(np,
NULL)), this index is still -1, and of_parse_phandle_with_args() will
return -EINVAL (This index will be updated if if it had clock name).
clk_get_by_name(np, NULL) should work, then, default index should be 0
instead of -1. This patch fixes it.

Fixes: 4472287a ("clk: Introduce of_clk_get_hw_from_clkspec()")
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>