net: qualcomm: rmnet: Add support for ethtool private stats for port

Add ethtool private stats handler to debug the handling of packets
with downlink header and trailer. This functionality is added
as part of a new private structure for statistics in the rmnet
port structure.

CRs-Fixed: 2233026
Change-Id: Iefd3687b7d61d314e363be556e71ac419b5119ea
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>