ASoC: stm: Use *-y instead of *-objs in Makefile

*-objs suffix is reserved rather for (user-space) host programs while
usually *-y suffix is used for kernel drivers (although *-objs works
for that purpose for now).

Let's correct the old usages of *-objs in Makefiles.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240507155540.24815-29-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/stm/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # SAI
-snd-soc-stm32-sai-sub-objs := stm32_sai_sub.o
+snd-soc-stm32-sai-sub-y := stm32_sai_sub.o
 obj-$(CONFIG_SND_SOC_STM32_SAI) += snd-soc-stm32-sai-sub.o
 
-snd-soc-stm32-sai-objs := stm32_sai.o
+snd-soc-stm32-sai-y := stm32_sai.o
 obj-$(CONFIG_SND_SOC_STM32_SAI) += snd-soc-stm32-sai.o
 
 # I2S
-snd-soc-stm32-i2s-objs := stm32_i2s.o
+snd-soc-stm32-i2s-y := stm32_i2s.o
 obj-$(CONFIG_SND_SOC_STM32_I2S) += snd-soc-stm32-i2s.o
 
 # SPDIFRX
-snd-soc-stm32-spdifrx-objs := stm32_spdifrx.o
+snd-soc-stm32-spdifrx-y := stm32_spdifrx.o
 obj-$(CONFIG_SND_SOC_STM32_SPDIFRX) += snd-soc-stm32-spdifrx.o
 
 #DFSDM