net: axienet: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding a fallthrough statement instead of just
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/20210528195831.GA39131@embeddedor


Signed-off-by: Jakub Kicinski <kuba@kernel.org>