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>