iwlwifi: mvm: wait for handlers when stopping scans

The recent unified scan api change introduced issues
when stopping ongoing scans, since both regular and
sched scan now use same stopped notification.

When issuing a new scan right after a running one,
we get the "old" notification and handle it wrongly
as notification for the current scan.

Fix it by introducing a new function that make sure
we consume the pending notifications before issuing
a new scan.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Reviewed-by: ArikX Nemtsov <arik@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>