mac80211: remove tx_frags driver callback

The implementation of tx_frags is buggy due to
not handling queue stop, and there's no driver
implementing it so remove it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/main.c

@@ -587,7 +587,7 @@ struct ieee80211_hw *ieee80211_alloc_hw(size_t priv_data_len,
 
 	local->hw.priv = (char *)local + ALIGN(sizeof(*local), NETDEV_ALIGN);
 
-	BUG_ON(!ops->tx && !ops->tx_frags);
+	BUG_ON(!ops->tx);
 	BUG_ON(!ops->start);
 	BUG_ON(!ops->stop);
 	BUG_ON(!ops->config);