multiboot2: clippy fixes only needed for Rust 1.70 #957

Job	Run time
build (msrv) / rust	18s
build (stable) / rust	11s
build (nightly) / rust	21s
build no_std (stable) / rust	15s
build no_std (stable) [w/o builder] / rust	22s
build no_std (stable) [Windows] / rust	56s
style (stable) / rust	15s
style (msrv) / rust	22s
build no_std (msrv) / rust	30s
build no_std (nightly) / rust	20s
tests with miri (nightly) / rust	1m 25s
style (nightly) / rust	22s
	5m 37s

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