iwlwifi: Cancel scanning upon association

This patch cancels an ongoing HW scan upon association. Not doing this
resulted in instability in association.

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