usb: gadget: u_ether: Add RMNET extended IOCTLs

Support IOCTL calls for setting/getting MRU, which is an
RMNET extension that can be set independently of MTU. This
value will limit the size of packets received from USB
OUT endpoint. Also add other required RMNET IOCTLs.

Change-Id: Ib8c50d6ac598095ccf881ad89c75337afc1bda0c
Signed-off-by: Jack Pham <jackp@codeaurora.org>