iwlagn: fix slot programming

When an AP mode interface is added with a DTIM
period of two, the slot programming is wrong.
Fix it by taking into account the DTIM period.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>