mac80211: clarify scan request

When a scan is requested for non-STA interfaces, we simply fire
off a scan, but for STA interfaces we shouldn't because they
could be in the middle of an association. This clarifies the
corresponding code.

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