drivers: wifi: siwx917: Set security configuration to default

- Updated Wi-Fi security configuration to use default, allowing
  the device to select the best available encryption.
- Removed unsupported security features.
- Replaced `WIFI_SECURITY_TYPE_SAE` with `WIFI_SECURITY_TYPE_SAE_AUTO`
  to enable automatic selection between SAE-HNP and SAE-H2E,
  as the device supports both modes by default.

Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>