mac80211: Fix ROC duration == 0 handling

In case the given ROC duration is 0, update it to a minimal value before
setting the ieee80211_roc_work parameters, so it also would be valid
for cases where scan is in progress or there are other ROCs queued.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>