wl1271: Fix to join and channel number handling

This patch changes the way JOIN's are performed, and channel numbers updated.
The reason for this is that the firmware JOIN command clears WPA(2) key
material, and if done while associated to a WPA(2) secured AP, will render
the data-path unusable.

While the channel is not usually changed while associated (and currently we
could not even support something like that), after performing a scan operation
while associated, mac80211 will re-set the current channel to the driver. This
caused our problem.

Also, the mac80211 is assuming that the driver channel configuration remains
persistent over periods of IDLE. Therefore remove channel resetting to zero
from the unjoin function.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>