ntb: ntb_transport: Mark expected switch fall-throughs

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

Addresses-Coverity-ID: 1373888 ("Missing break in switch")
Addresses-Coverity-ID: 1373889 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Allen Hubbe <allenbh@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>