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

Addresses-Coverity-ID: 1056537 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Khalid Aziz <khalid@gonehiking.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>