ASoC: rt286: Fix run time error while modifying const data

Make a copy of memory for index_cache rather than directly use the
rt286_index_def to avoid run time error.

Fixes: c418a84a

 ("ASoC: Constify reg_default tables")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>