nl80211: Fix fall-through warnings 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>
Link: https://lore.kernel.org/r/fe5afd456a1244751177e53359d3dd149a63a873.1605896060.git.gustavoars@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -11175,6 +11175,7 @@ static int nl80211_tx_mgmt(struct sk_buff *skb, struct genl_info *info)
 	case NL80211_IFTYPE_P2P_DEVICE:
 		if (!info->attrs[NL80211_ATTR_WIPHY_FREQ])
 			return -EINVAL;
+		break;
 	case NL80211_IFTYPE_STATION:
 	case NL80211_IFTYPE_ADHOC:
 	case NL80211_IFTYPE_P2P_CLIENT: