powerpc/cell: Use fallthrough;

Convert the various uses of fallthrough comments to fallthrough;

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/03073a9a269010ca439e9e658629c44602b0cc9f.1583896348.git.joe@perches.com