ASoC: rsnd: adg: care clock-frequency size

ADG has buffer over flow bug if DT has more than 3 clock-frequency.
This patch fixup this issue, and uses first 2 values.

	clock-frequency = <x y>;	/* this is OK */
	clock-frequency = <x y z>;	/* this is NG */

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>