cfg80211: validate AID of stations being added

We have some validation code in mac80211 but said code will
force an invalid AID to 0 which isn't a valid AID either;
instead require a valid AID (1-2007) to be passed in from
userspace in cfg80211 already. Also move the code before
the race comment since it can only be executed during STA
addition and thus is not racy.

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