ASoC: ep93xx: ignore 0 Hz sysclk

Commit 2458adb8
("SoC: simple-card-utils: set 0Hz to sysclk when shutdown")
added a call to snd_soc_dai_set_sysclk() with 0 Hz frequency. Being
propagated further it causes a division by zero in clk-ep93xx driver:

Division by zero in kernel.
CPU: 0 PID: 52 Comm: aplay Tainted: G        W          6.2.0-rc4-... #1
Hardware name: Generic DT based system
 unwind_backtrace from show_stack+0x10/0x18
 show_stack from dump_stack_lvl+0x28/0x34
 dump_stack_lvl from __div0+0x10/0x1c
 __div0 from Ldiv0+0x8/0x1c
 Ldiv0 from ep93xx_mux_determine_rate+0x78/0x1d0
 ep93xx_mux_determine_rate from clk_core_round_rate_nolock+0x48/0xc8
 clk_core_round_rate_nolock from clk_core_set_rate_nolock+0x48/0x160
 clk_core_set_rate_nolock from clk_set_rate+0x30/0x8c
 clk_set_rate from ep93xx_i2s_set_sysclk+0x30/0x6c
 ep93xx_i2s_set_sysclk from snd_soc_dai_set_sysclk+0x3c/0xa4
 snd_soc_dai_set_sysclk from asoc_simple_shutdown+0xb8/0x164
 asoc_simple_shutdown from snd_soc_link_shutdown+0x44/0x54
 snd_soc_link_shutdown from soc_pcm_clean+0x78/0x180
 soc_pcm_clean from soc_pcm_close+0x28/0x40
 soc_pcm_close from snd_pcm_release_substream.part.0+0x3c/0x84
 snd_pcm_release_substream.part.0 from snd_pcm_release+0x40/0x88
 snd_pcm_release from __fput+0x74/0x278

There has been commit f1879d7b ("ASoC: rockchip: ignore 0Hz sysclk"),
but it prepared by far not all drivers.
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Link: https://lore.kernel.org/r/20230212220923.258414-1-alexander.sverdlin@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/cirrus/ep93xx-i2s.c

@@ -359,6 +359,8 @@ static int ep93xx_i2s_set_sysclk(struct snd_soc_dai *cpu_dai, int clk_id,
 
 	if (dir == SND_SOC_CLOCK_IN || clk_id != 0)
 		return -EINVAL;
+	if (!freq)
+		return 0;
 
 	return clk_set_rate(info->mclk, freq);
 }