net: usb: Implement new IOCTLs for RmNET data driver

RmNET data driver interacts with underlying RmNET HSIC
transport driver by issuing following new IOCTLs

RMNET_IOCTL_GET_SUPPORTED_FEATURES: Get the transport
driver supported features.
RMNET_IOCTL_SET_MRU: Set the MRU size.
RMNET_IOCTL_GET_MRU: Return the currently configured
MRU.
RMNET_IOCTL_GET_DRIVER_NAME: Get the underlying transport
peripheral driver name.

Change-Id: If52281e630d02be895beee8b43d122ea54e6d625
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>