nl80211: Allow association to change channels during reassociation

nl80211_associate() was rejecting (re)association attempts with EBUSY
in some cases where we are currently associated with an AP that uses
different channel from the destination AP. Fix this by passing the
current wdev to rdev_fixed_channel() in the same way that was already
done for join-IBSS and connect commands. This allows the fixedchan
check to skipped for the current wdev and allows the reassociation to
proceed.

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