iwlwifi: mvm: Fix skip over dtim configuration in d0i3

There was some confusion concerning the units of the beacon interval.
The driver assumed that it was in msec where it was in TU - so fix that.
Skip over dtim was capped by 300TU where it should be by 306TU.
It should also be subjected to several conditions:
Not a DFS channel, dtim period < 10, and the multicast wake-lock
is off.  Concerning multicast lock - there is an implementation gap
in the supplicant, so just leave a TODO.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>