carl9170: Only specify interface combinations if more than one interface is possible

Otherwise carl9170 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))

Note: The number of supported interfaces is set by
the carl9170 firmware. The default number of
supported interfaces for all current firmwares is 2.
Therefore this warning can only be observed with
custom firmwares.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>