rndis_wlan: prevent integer overflow in indication()

If we pick a high value for "offset" then it could lead to an integer
overflow and we would get past the check for:
	if (offset + len > buflen) { ...

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>