cfg80211: Allow reassociation in associated state

cfg80211 rejects all association requests when in associated state. This
prevents clean roaming within an ESS since one would first need to
disassociate before being able to request reassociation.

Accept the reassociation request and let the old association to be
dropped when the new one is completed. This fixes nl80211-based
roaming with the current snapshot version of wpa_supplicant (that has
code for requesting reassociation explicitly withthe previous BSSID
attribute).

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>