cfg80211: fix channel setting for wext

Patch fixes the bug at
http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2139



Currently we cannot set the channel using wext extension
if we have already associated and disconnected. As
cfg80211_mgd_wext_siwfreq will not switch the channel if ssid is set.
This fixes it by clearing the ssid.
Following is the sequence which it tries to fix.
modprobe iwlagn
iwconfig wlan0 essid ""
ifconfig wlan0 down
iwconfig wlan0 chan X

wext is marked as deprecate.If we use nl80211 we can easily play with
setting the channel.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>