drivers: wifi: siwx91x: Fix default scan time handling

The commit `1f73ca3` clarified that a scan time of 0 should be used to
apply the default scan timeout, as opposed to using
SL_WIFI_DEFAULT_ACTIVE_CHANNEL_SCAN_TIME, which incorrectly results in
a 65-second timeout.

However, this fix was unintentionally omitted in commit `bf22b61`
when the `sl_si91x_configure_timeout()` function was relocated.

This commit restores the behavior.

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