cfg80211: separate internal SME implementation

The current internal SME implementation in cfg80211 is
very mixed up with the MLME handling, which has been
causing issues for a long time. There are three things
that the implementation has to provide:
 * a basic SME implementation for nl80211's connect()
   call (for drivers implementing auth/assoc, which is
   really just mac80211) and wireless extensions
 * MLME events for the userspace SME
 * SME events (connected, disconnected etc.) for all
   different SME implementation possibilities (driver,
   cfg80211 and userspace)

To achieve these goals it isn't necessary to track the
software SME's connection status outside of it's state
(which is the part that caused many issues.) Instead,
track it only in the SME data (wdev->conn) and in the
general case only track whether the wdev is connected
or not (via wdev->current_bss.)

Also separate the internal implementation to not have
callbacks from the SME events, but rather call it from
the API functions that the driver (or rather mac80211)
calls. This separates the code better.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>