ASoC: rsnd: adg: use more simple method for null_clk

commit 965386c9 ("ASoC: rsnd: call unregister for null_hw when
removed") tried unregister null_clk, but it has some issues.

1st issue is kernel will indicate below message when unregistering,
because of its timing. unregistering should be happen after clk_disable().

	clk_unregister: unregistering prepared clock: rsnd_adg_null

2nd issue is, it is using priv->null_clk, but it should be adg->null_clk.

3rd issue is it is using very complex clk registering method.
more simple clk_register/unregister_fixed_rate() should be OK.

This patch fixes these.

Fixes: 965386c9

 ("ASoC: rsnd: call unregister for null_hw when removed")
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87r1hli215.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>