kgdb: Fix fall-through warning for Clang

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

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210528200222.GA39201@embeddedor


Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>