ASoC: sam9g20ek: Disable capture unless building with microphone input

Without modification the AT91SAM9G20-EK has no capture support, none of the
inputs of the CODEC are wired to anything to useful and there are no paths
supporting loopback. Since the audio is clocked from the CODEC and the DAPM
inputs are marked as unusable this means that capture will fail to transfer
any data as the ADC path can't be powered up.

Flag this in the device description so apps don't see unusable capture
support, guarded with the existing optional define for mic input.

Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230225-asoc-sam9g20ek-v1-1-9baeb4893142@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/atmel/sam9g20_wm8731.c

@@ -98,6 +98,9 @@ static struct snd_soc_dai_link at91sam9g20ek_dai = {
 	.init = at91sam9g20ek_wm8731_init,
 	.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF |
 		   SND_SOC_DAIFMT_CBP_CFP,
+#ifndef ENABLE_MIC_INPUT
+	.playback_only = true,
+#endif
 	SND_SOC_DAILINK_REG(pcm),
 };