net: rmnet_data: Set the protocol when sending MAP ACK's

Set the protocol as ETH_P_MAP when sending a MAP ACK as a response
to a MAP command packet. Physical device drivers may drop these
packets if the ETH_P_MAP protocol is not present on the skb.

CRs-Fixed: 961336
Change-Id: I05ca1350fb16747b0300106654a74d3318389a30
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>