ASoC: rsnd: call unregister for null_hw when removed

commit d6956a7d ("ASoC: rsnd: add null CLOCKIN support")
added null_clk, but it is using local static valuable.
It will be leaked if rsnd driver was removed.
This patch moves it to priv, and call unregister when removing.

Fixes: d6956a7d

 ("ASoC: rsnd: add null CLOCKIN support")
Link: https://lore.kernel.org/r/87tumsoe2p.wl-kuninori.morimoto.gx@renesas.com
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Message-Id: <877djknbl5.wl-kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>