wext: Add cfg80211-specific optimization to avoid silly behavior

wext: Add cfg80211-specific optimization to avoid silly behavior

If the driver is detected to use cfg80211, we can rely on it being able
to disconnect with SIOCSIWMLME commands and to use empty SSID as a way
to stop it from associating when we are in progress of configuring the
driver for association. Consequently, we can remove the hack that uses
random 32-octet SSID to force disconnection and re-order association
commands to match the expectations that cfg80211 has for WEXT ioctls.
This gets rid of extra scan rounds and attempts to associate with the
silly 32-octet SSID.