net: usb: Define waitqueue head as static global variable

usbnet_terminate_urbs() defines waitqueue head as local variable.
There is a possibility that wake_up is called on this queue head
in workqueue context and usbnet_terminate_urbs() already returned
or dev->wait pointer becomes NULL while calling wake_up() from
workqueue context. Hence call wake_up() using waitqueue head
defined as static global variable.

CRs-Fixed: 388724
Change-Id: Ib7e6c3023b9d780af0466496b25e68113b2d3fd3
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>