usb: gadget: u_ether: Add support for ETH_P_MAP skb protocol type

In the case of RMNET IP mode and a packet is neither IPv4
nor IPv6, check if it supports ETH_P_MAP protocol type.
This allows upper layers to use the MAP protocol to handle
MUXing and de-MUXing of the skb accordingly.

Change-Id: I04f9bf3ad11d6dafcbeb271505f50b571148cb6c
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>