cfg80211: prohibit scanning the same channel more than once

It isn't very useful to scan the same channel more than once
during a given scan, and some hardware (notably iwlwifi) can
only scan a limited number of channels at a time. To prevent
any overflows, simply disallow scanning any channel multiple
times in a given scan command. This is a small change in the
userspace ABI, but the only user, wpa_supplicant, never asks
for a scan with the same frequency listed twice.

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