iwlwiwi: mvm: use bss_conf->dtim_period instead of conf.ps_dtim_period

The latter is meant for software implementation of power
save and is not per-virtual interface. Since our driver
supports multiple virtual interfaces, we need to use
vif->bss_conf.dtim_period.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>