clk: Fix falling back to legacy parent string matching

clk: Fix falling back to legacy parent string matching

Calls to clk_core_get() will return ERR_PTR(-EINVAL) if we've started
migrating a clk driver to use the DT based style of specifying parents
but we haven't made any DT updates yet. This happens when we pass a
non-NULL value as the 'name' argument of of_parse_clkspec(). That
function returns -EINVAL in such a situation, instead of -ENOENT like we
expected. The return value comes back up to clk_core_fill_parent_index()
which proceeds to skip calling clk_core_lookup() because the error
pointer isn't equal to -ENOENT, it's -EINVAL.

Furthermore, we blindly overwrite the error pointer returned by
clk_core_get() with NULL when there isn't a legacy .name member
specified in the parent map. This isn't too bad right now because we
don't really care to differentiate NULL from an error, but in the future
we should only try to do a legacy lookup if we know we might find
something. This way DT lookups that fail don't try to lookup based on
strings when there isn't any string to match, hiding the error from DT
parsing.

Fix both these problems so that clk provider drivers can use the new
style of parent mapping without having to also update their DT at the
same time. This patch is based on an earlier patch from Taniya Das which
checked for -EINVAL in addition to -ENOENT return values from
clk_core_get().

Fixes: 601b6e93304a ("clk: Allow parents to be specified via clkspec index")
Cc: Taniya Das <tdas@codeaurora.org>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Reported-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190813214147.34394-1-sboyd@kernel.org
Tested-by: Taniya Das <tdas@codeaurora.org>