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

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: 1a74b21ce59f ("ASoC: SOF: amd: Add Probe functionality support for amd platforms.")
Fixes: 9c2f5b6eb8b7 ("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.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>