rxrpc: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>