Staging: gdm72xx: underflow bug in gdm_wimax_ioctl_get_data()

"size" here should be unsigned, otherwise we might end up trying to copy
negative bytes in gdm_wimax_ioctl_get_data() resulting in an information
leak.

Reported-by: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>