mac80211: fix remain-on-channel cancellation

Ilan's previous commit 1b894521

 ("mac80211: handle HW
ROC expired properly") neglected to take into account that
hw_begun was now always set in the software implementation
as well as the offloaded case.

Fix hw_begun to only apply to the offloaded case to make
the check in Ilan's commit safe and correct.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>