USB: usbdevfs_urb: __user annotation

fix warning:
drivers/usb/core/devio.c:1226:20: warning: incorrect type in assignment (different address spaces)
drivers/usb/core/devio.c:1226:20:    expected void *usercontext
drivers/usb/core/devio.c:1226:20:    got void [noderef] <asn:1>*

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>