UBSan: Avoid unsigned integer overflow is throughput estimation

UBSan: Avoid unsigned integer overflow is throughput estimation

wpa_scan_result_compar() would return wb->est_throughput -
wa->est_throughput in case the comparison is done based on the
throughput estimates. While the return value from this function is a
signed integer, these est_throughput values are unsigned integers and
need to be explicitly typecast to avoid an UBSan warning.

scan.c:1996:30: runtime error: unsigned integer overflow: 54000 - 135000 cannot be represented in type 'unsigned int'

Signed-off-by: Jouni Malinen <j@w1.fi>