mac80211_hwsim: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/net/wireless/mac80211_hwsim.c: In function ‘init_mac80211_hwsim’:
drivers/net/wireless/mac80211_hwsim.c:3853:21: warning: this statement may fall through [-Wimplicit-fallthrough=]
    param.reg_strict = true;
    ~~~~~~~~~~~~~~~~~^~~~~~
drivers/net/wireless/mac80211_hwsim.c:3854:3: note: here
   case HWSIM_REGTEST_DRIVER_REG_ALL:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>