Use size_t instead of unsigned_int for last_scan_res

Use size_t instead of unsigned_int for last_scan_res

This avoids a theoretical unsigned integer overflow case with 32-bit
integers, but something that could potentially be hit with 16-bit int
(though, even that part looks pretty theoretical in this particular case
of number of BSSs in scan results).

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