ASoC: SOF: amd: Fix build error with built-in config

Makefile in AMD ACP driver has a line substitution with "=" instead of
"+="; this overrides the preexisting item, hence it broke the build
after the recent change to replace *-objs with *-y.

This patch corrects the line.

Fixes: 1a74b21c ("ASoC: SOF: amd: Add Probe functionality support for amd platforms.")
Fixes: 9c2f5b6e ("ASoC: SOF: Use *-y instead of *-objs in Makefile")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20240510170305.03b67d9f@canb.auug.org.au
Link: https://lore.kernel.org/r/20240510073656.23491-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/soc/sof/amd/Makefile

@@ -5,7 +5,7 @@
 # Copyright(c) 2021, 2023 Advanced Micro Devices, Inc. All rights reserved.
 
 snd-sof-amd-acp-y := acp.o acp-loader.o acp-ipc.o acp-pcm.o acp-stream.o acp-trace.o acp-common.o
-snd-sof-amd-acp-$(CONFIG_SND_SOC_SOF_ACP_PROBES) = acp-probes.o
+snd-sof-amd-acp-$(CONFIG_SND_SOC_SOF_ACP_PROBES) += acp-probes.o
 snd-sof-amd-renoir-y := pci-rn.o renoir.o
 snd-sof-amd-rembrandt-y := pci-rmb.o rembrandt.o
 snd-sof-amd-vangogh-y := pci-vangogh.o vangogh.o