mac80211: combine roc with the "next roc" if possible

If the remaining time in the current roc is not long
enough, mac80211 adds the new roc right after it
(if they have similar params).

However, in case of multiple rocs, the "next roc"
is not considered, resulting in multiple rocs,
each one with its own duration.

Refactor the code a bit and consider the next roc,
so a single max roc will be used instead of
multiple rocs (which might last much longer).

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>