usb: gadget: u_ether: Add 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: Icaf70f52b15e642aefa6a4c6e5494dae435aa7f3
Signed-off-by: Jack Pham <jackp@codeaurora.org>