ASoC: ad193x: Drop .volatile_reg implementation

adau193x_reg_volatile() always return false.
This seems pointless because current code uses REGCACHE_NONE cache_type
which is supposed to be volatile.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>