net: rmnet_data: Set SKB protocol for outbound MAP packets

Set the skb->protocol field to ETH_P_MAP for egress packets which
have MAP enabled. Required for lower level drivers to ensure that
only MAP packets are being transmitted.

CRs-Fixed: 554883
Change-Id: I6fa852344ef36e079cc610cbed152555aae9d6f2
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>