clk: qcom: regmap-mux: fix parent clock lookup

The function mux_get_parent() uses qcom_find_src_index() to find the
parent clock index, which is incorrect: qcom_find_src_index() uses src
enum for the lookup, while mux_get_parent() should use cfg field (which
corresponds to the register value). Add qcom_find_cfg_index() function
doing this kind of lookup and use it for mux parent lookup.

Fixes: df964016

 ("clk: qcom: add parent map for regmap mux")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20211115233407.1046179-1-dmitry.baryshkov@linaro.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>