cfg80211: use reassociation when possible

cfg80211: use reassociation when possible

With the move of everything related to the SME from
mac80211 to cfg80211, we lost the ability to send
reassociation frames. This adds them back, but only
for wireless extensions. With the userspace SME, it
shall control assoc vs. reassoc (it already can do
so with the nl80211 interface).

I haven't touched the connect() implementation, so
it is not possible to reassociate with the nl80211
connect primitive. I think that should be done with
the NL80211_CMD_ROAM command, but we'll have to see
how that can be handled in the future, especially
with fullmac chips.

This patch addresses only the immediate regression
we had in mac80211, which previously sent reassoc.

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