In rtw_mlme_ext.c, falling through makes perfect sense. I've
there replaced the ad hoc comment with the necessary gcc
attribute. In rtw_ap.c, the default case seems to agree with
the previous ones only by happenstance, so I've inserted an
actual break there. The warnings no longer fire.
Yes, there's a portable [[fallthrough]] -- for C++17. This
won't work on Clang, but neither will the kernel, nor this
Makefile.
