sh/cpu: Make sure play_dead() doesn't return

play_dead() doesn't return.  Make that more explicit with a BUG().

BUG() is preferable to unreachable() because BUG() is a more explicit
failure mode and avoids undefined behavior like falling off the edge of
the function into whatever code happens to be next.

Link: https://lore.kernel.org/r/d0c3ff5349adfe8fd227acc236ae2c278a05eb4c.1676358308.git.jpoimboe@kernel.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>