iwlagn: fix PAN slot timing wrt. DTIM

When the DTIM is not 1, then the slot timing is in
some cases required to be calclulated based on the
DTIM interval instead of the beacon interval, fix
that.

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