In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This addresses Coverity-ID: 114713 ("Missing break in switch").
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>