ASoC: rt298: Make rt298_index_def const

The index_cache is per instance run time state but rt298_index_def is not.
Make rt298_index_def const and make a copy of memory for index_cache rather
than directly use the rt298_index_def.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>