wifi: iwlwifi: mvm: adjust skip-over-dtim in D3

The current formula can skip both too much and not enough time,
given the +1 (where the comment about firmware is wrong). Adjust
the formula accordingly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230620125813.32406b6828ae.I88c315b85f7c56ac6109f84580b95a3dd104ff6c@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>