mac80211: flush hw_roc_start work before cancelling the ROC

When HW ROC is supported it is possible that after the HW notified
that the ROC has started, the ROC was cancelled and another ROC was
added while the hw_roc_start worker is waiting on the mutex (since
cancelling the ROC and adding another one also holds the same mutex).
As a result, the hw_roc_start worker will continue to run after the
new ROC is added but before it is actually started by the HW.
This may result in notifying userspace that the ROC has started before
it actually does, or in case of management tx ROC, in an attempt to
tx while not on the right channel.

In addition, when the driver will notify mac80211 that the second ROC
has started, mac80211 will warn that this ROC has already been
notified.

Fix this by flushing the hw_roc_start work before cancelling an ROC.

Cc: stable@vger.kernel.org
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>