r8169: Fix fall-through warning 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


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/20210528202327.GA39994@embeddedor


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