rt2x00: Only specify interface combinations if more then one interface is possible

Otherwise rt2500* triggers a warning in cfg80211, from net/wireless/core.c:

	/* Combinations with just one interface aren't real */
	if (WARN_ON(c->max_interfaces < 2))

This was introduced in commit 55d2e9da:
rt2x00: Replace open coded interface checking with interface combinations.
Reported-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: stable@vger.kernel.org [3.7+]
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00dev.c

@@ -1124,6 +1124,9 @@ static inline void rt2x00lib_set_if_combinations(struct rt2x00_dev *rt2x00dev)
 	struct ieee80211_iface_limit *if_limit;
 	struct ieee80211_iface_combination *if_combination;
 
+	if (rt2x00dev->ops->max_ap_intf < 2)
+		return;
+
 	/*
 	 * Build up AP interface limits structure.
 	 */