ASoC: tegra: Fix uninitialized flat cache warning in tegra210_ahub

ASoC: tegra: Fix uninitialized flat cache warning in tegra210_ahub

The tegra210_ahub driver started triggering a warning after commit
e062bdfdd6ad ("regmap: warn users about uninitialized flat cache"),
which flags drivers using REGCACHE_FLAT without register defaults.
Since the driver omits default definitions because its registers are
zero initialized, the following warning is shown:

  WARNING KERN tegra210-ahub 2900800.ahub: using zero-initialized flat cache, this may cause unexpected behavior

Switch to REGCACHE_FLAT_S which is the recommended cache type for
sparse register maps without defaults. This cache type initializes
entries on-demand from hardware, eliminating the warning while using
memory efficiently.

Signed-off-by: sheetal <sheetal@nvidia.com>
Link: https://patch.msgid.link/20251208052040.4025612-1-sheetal@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>