ASoC: dwc: make pcm support built-in when necessary

The new PIO mode for the dwc audio driver causes a link failure
when it is built as a loadable module but the audio driver is built-in:

sound/built-in.o: In function `i2s_irq_handler':
:(.text+0x58c64): undefined reference to `dw_pcm_push_tx'
sound/built-in.o: In function `dw_i2s_probe':
:(.text+0x593dc): undefined reference to `dw_pcm_register'

We could link both into a single module, but apparently the
author intended them to be separate, so this instead changes
the Makefile to force the pcm module to be built-in if the
base module is. This is a bit hacky but not as bad as trying
to work around it in Kconfig language.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 79361b2b ("ASoC: dwc: Add PIO PCM extension")
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/dwc/Makefile

 # SYNOPSYS Platform Support
 obj-$(CONFIG_SND_DESIGNWARE_I2S) += designware_i2s.o
-obj-$(CONFIG_SND_DESIGNWARE_PCM) += designware_pcm.o
-
+ifdef CONFIG_SND_DESIGNWARE_PCM
+obj-$(CONFIG_SND_DESIGNWARE_I2S) += designware_pcm.o
+endif