ASoC: codecs: ad193x: Set constraint to always have 32 sample bits

DACs and ADCs on ad193x codecs require a 32 bit slot size. We should
assure that no other size is used.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>