mac80211: fix cfg80211 hooks for master interface

The master interface is a virtual interface that is registered
to mac80211, changing that does not seem like a good idea at
the moment. However, since it has no sdata, we cannot accept
any configuration for it. This patch makes the cfg80211 hooks
reject any such attempt.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>