iwlwifi: use dtim_period from association, and set listen_interval

This patch uses dtim_period from association, and sets the listen_interval.

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