usb: gadget: u_ether: prepare for NCM

NCM is a Network Control Model, subclass of USB CDC class,
specification is available at http://www.usb.org/developers/devclass_docs

This patch makes possible for u_ether to use multiply of wMaxPacketSize
predefined size transfers without ZLP (Zero Length Packet), required
by NCM spec.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>