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Ensure that the default (zero) value represents the case where we take
no action. Previously, if a PLAT=sun50i_a64 build was booted on an
unknown SoC ID, it would be treated as an H5 at shutdown.

This removes some duplicate code and fixes error propagation on H6.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: I4e51d8a43a56eccb0d8088593cb9908e52e782bc