iwlwifi: don't override association channel with control channel

This patch fixes override of association channel with HT control channel.
The scenario is currently happening because disassociation flow
doesn't clean previous association information (such as is_ht and control
channel).

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>