Fix handling of CRC start and valid signals #3586

Job	Run time
check-links	53s
smoke (amaranth-lang/amaranth-boards)	33s
test (3.9)	2m 48s
smoke (amaranth-lang/amaranth-stdio)	31s
test (3.10)	2m 11s
smoke (amaranth-lang/amaranth-soc)	30s
test (3.11)	2m 53s
document	52s
test (3.12)	3m 52s
test (3.13)	3m 7s
test (pypy-3.9)	6m 54s
test (pypy-3.10)	4m 45s
test (3.14-dev)	1m 48s
publish-docs	9s
publish-docs-dev	0s
required	1s
publish-schemas	5s
publish-package	40s
publish-release	0s
	32m 32s

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