mac80211: adjust roc duration when combining ROCs

The new duration (remaining duration after the current
ROC ends) was calculated but not used, making the
optimization worthless.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>