wifi: rtw89: purge obsoleted scan events with software sequence number

The queued and obsoleted scan events can be wrongly treated as events of
new scan request, causing unexpected scan result. Attach a software
sequence number to scan request and its corresponding events. When a new
scan request is acknowledged by firmware, purge the scan events if its
sequence number is not belong to current request.

Normal case:

   mac80211                   event work        event BH
   -------------              ----------        --------
   scan req #1 ---->o
                    |
               <----o  <...........................o
                                  o
                                  |
       <--------------------------+
         ieee80211_scan_completed()

Abnormal case (late event work):

   mac80211                   event work        event BH
   -------------              ----------        --------
   scan req #1 ---->o
                    |
               <----o  <...........................o
                                  o #1

   scan cancel #2 ->o
                    |
               <----o  <...........................o
                                  o #2
                                  | (patch to avoid this)
   scan req #3 ---->o             |
                    |             |
               <----o  <..........|................o
                                  | o #3
       <--------------------------+
         ieee80211_scan_completed()

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250715035259.45061-5-pkshih@realtek.com