driver: wifi: siwx91x: Add configurable maximum BSS scan

This commit introduces support for limiting the number of BSS scan
results returned by the siwx91x Wi-Fi driver. If scan_max_bss_cnt is
specified, the driver will return up to the specified count of BSS
results. If not specified, the driver defaults to returning all
available results.

Co-authored-by: Arunmani Alagarsamy <arunmani.a@silabs.com>
Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>