net: qualcomm: rmnet: Add support to reset ethtool private stats

Add ethtool private stats handler to reset the statistics of
handling of packets with downlink header and trailer.
“ethtool -r rmnet_data0” will reset the stats.

CRs-Fixed: 2285862
Change-Id: Ib3da8c2b7b23d72de2795f93a90e8c0e3b2ce869
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>