driver: wifi: siwx91x: Fix scan timeout

WiseConnect documentation says:

  Default value of 100 millisecs is used when
  SL_WIFI_DEFAULT_ACTIVE_CHANNEL_SCAN_TIME is passed".

However, this is not true. Currently, DEFAULT_ACTIVE_CHANNEL_SCAN_TIME
is 0xFFFF and the scan time is set to 65 seconds.

To have the default scan time, the value '0' must be used.

Fortunately, '0' is also the value we get when the user does not specify
any default scan time. So the code can be simplified.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>