mac80211: Fix reassociation processing (within ESS roaming)

mac80211: Fix reassociation processing (within ESS roaming)

Commit e1dd33f60ced091114e4aacf141e0d03b88d3e13 changed cfg80211 to
allow association commands while in associated state to enable support
for roaming within an ESS. However, this was not enough to resolve all
cases with mac80211 which needs some additional handling of the
reassociation case to clear internal state with the BSS that was in use
previously.

This patch makes ieee80211_mgd_assoc() accept a valid reassociation
command and clean the association state with the previous BSS. This
fixes roaming between BSSes in an ESS when using wpa_supplicant with
-Dnl80211.

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