iwlwifi: mvm: abort scan on sched_scan request

A scheduled scan is a more persistent setting and should take priority
over temporary regular scans. Abort the regular when a sched_scan
request arrives and then request the sched_scan.

The kernel API allows sending a sched_scan without canceling a regular
scan in progress, so this is our way to abstract the FW's limitations.

Make the scan-cancel Rx handler async and flush after invocation to
ensure new scans can't creep in before it.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>