net: wifi: shell: only process scan events during requested scan

[EricNRS]

The scan events are always enabled which means if another software component requests a scan, then the WiFi shell scan printouts will also be sent to the shell. This change only enables the network management scan events when a user has requested a scan.

Without the change when my application does a scan to find the best channel:

uart> config wifi_channel
Scanning for best channels
SSID IoT, Channel 11, RSSI -46 dBm
SSID IoT, Channel 11, RSSI -63 dBm
SSID IoT, Channel 6, RSSI -77 dBm
SSID IoT, Channel 1, RSSI -81 dBm
Scan completed
Best Channel: 11, Max RSSI: -46 dBm

# These are printed from the WiFi Shell even though it wasn't requested:
Num  | SSID                             (len) | Chan (Band)   | RSSI | Security        | BSSID             | MFP     
1    | IoT                              3     | 11   (2.4GHz) | -46  | WPA2-PSK        | -------------------- | Disable 
2    | IoT                              3     | 11   (2.4GHz) | -63  | WPA2-PSK        | --------------------  | Disable 
3    | IoT                              3     | 6    (2.4GHz) | -77  | WPA2-PSK        | --------------------  | Disable 
4    | IoT                              3     | 1    (2.4GHz) | -81  | WPA2-PSK        | --------------------  | Disable 
Scan request done

[EricNRS]

The change is ready to merge, but marking as a draft since I have not discussed it with anyone yet.