clk: Do a DT parent lookup even when index < 0

We want to allow the parent lookup to happen even if the index is some
value less than 0. This may be the case if a clk provider only specifies
the .name member to match a string in the "clock-names" DT property. We
shouldn't require that the index be >= 0 to make this use case work.

Fixes: 601b6e93 ("clk: Allow parents to be specified via clkspec index")
Reported-by: Alexandre Mergnat <amergnat@baylibre.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>