wusb: handle wusb device ep0 speed settings

This patch teaches the USB stack handling of WUSB devices (those whose
speed is USB_SPEED_VARIABLE). For these devices, we need to set ep0's
maxpacketsize to 512 (even though the device descriptor reports it as
0xff).

New code being pushed to linuxuwb.org requires this patch to connect WUSB
devices.

Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>