ASoC: nau8822: Remove unused member from struct

Remove unused mclk_idx struct member.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://msgid.link/r/20240404111953.99287-1-francesco@dolcini.it


Signed-off-by: Mark Brown <broonie@kernel.org>