rtl8xxxu: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix
multiple warnings by replacing /* fall through */ comments with
the new pseudo-keyword macro fallthrough; instead of letting the
code fall through to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

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


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210305094850.GA141221@embeddedor