mac80211: use bss_conf->dtim_period instead of conf.ps_dtim_period

sta_set_sinfo is obviously takes data for specific station.
This specific station is attached to a specific virtual
interface. Hence we should use the dtim_period from this
virtual interface rather than the system wide dtim_period.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>