iwlwifi: only check for association id when associating with AP

There is no association process in IBSS mode - so testing the
association id is not needed.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
CC: Richard Scherping <richard@scherping.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>