Staging: gdm72xx: silence underflow warning in netlink_send()

The value of "group" comes from "idx" in __gdm_wimax_event_send():

	if (sscanf(e->dev->name, "wm%d", &idx) == 1)

Smatch marks sscanf values as user controlled.  It's supposed to be a
number in 0-30 range.  We cap the upper bound but allow negatives.  Fix
this by making it type u16 instead.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>