ASoC: stm32: select IIO_BUFFER

ASoC: stm32: select IIO_BUFFER

The new stm32_adfsdm driver has incomplete dependencies as shown
by Kconfig:

warning: (SND_SOC_STM32_DFSDM && LMP91000) selects IIO_BUFFER_CB which has unmet direct dependencies (IIO && IIO_BUFFER)
sound/soc/stm/stm32_adfsdm.o: In function `stm32_adfsdm_trigger':
stm32_adfsdm.c:(.text+0x8c): undefined reference to `stm32_dfsdm_get_buff_cb'
stm32_adfsdm.c:(.text+0x9c): undefined reference to `stm32_dfsdm_release_buff_cb'

This makes the dependency on SND_SOC_STM32_DFSDM unconditional, so we can
always resolve the stm32_dfsdm_get_buff_cb/stm32_dfsdm_release_buff_cb
symbols and get the implied IIO_BUFFER dependency.

compile-testing on other platforms is still possible as long as that IIO
driver is there.

Fixes: 55da094824c4 ("ASoC: stm32: add DFSDM DAI support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>