usb: gadget: u_ether: Add ethernet and rawip netdevice ioctls

Add ethernet device ioctls to switch between ethernet and raw IP
protocols modes. Add extended ioctls to set and get MRU size.
These ioctls are required to support RMNET over usb ethernet.

Change-Id: If38b442ace6de3f27e0c1a59684e295956af5c7d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>