soundwire: only handle alert events when the peripheral is attached #8178

Job	Run time
GCC build X86_64 with W=1 allmodconfig	1m 51s
GCC build X86_64 with W=1	3m 11s
GCC build X86_64	19m 37s
GCC static -fanalyzer	3m 57s
GCC build arm64 allmodconfig	1m 53s
GCC build arm64	23m 2s
Sparse check	2m 56s
Clang build x86_64	4m 52s
GCC build i386	16m 40s
	1h 17m 59s

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