rtlwifi: mark expected switch fall-through in rtl_make_smps_action

This is merged by Ping-Ke Shih from commit 640019bb ("staging:
rtlwifi: mark expected switch fall-through in rtl_make_smps_action"),
and original commit log is reserved below.

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>