Fix get_stas() bug

[heeres]

• cyw43_ll_wifi_ap_get_stas does not perform checks on the return value of cyw43_get_ioctl. On error (e.g. timeout), invalid num_stas values can be used in the following memcpy call, leading to crashes
• cyw43_ll_wifi_ap_get_stas now returns the return code of cyw43_do_ioctl on error
• Perform bounds checks before using memcpy. This requires the parameter num_stas to contain the size (in MAC addresses) of the supplied buffer.
• Associated functions in cyw43_ctrl.c set num_stas to 0 on error

[dpgeorge]

Thanks for the fix!