ASoC: rsnd: NULL check is not needed for clk_unprepare()

clk_unprepare() is checking parameter by IS_ERR_OR_NULL().
clk NULL check is not needed on rsnd_mod_quit()

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>