staging: rtl8192u: fix ->ssid overflow in r8192_wx_set_scan()

We need to cap len at IW_ESSID_MAX_SIZE (32) to avoid memory corruption.
This can be controlled by the user via the ioctl.

Fixes: 5f53d8ca

 ("Staging: add rtl8192SU wireless usb driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/YEHoAWMOSZBUw91F@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>