gianfar: Fix Rx byte accounting for ndev stats

[ Upstream commit 590399ddf9561f2ed0839311c8ae1be21597ba68 ]

Don't include in the Rx bytecount of the packet sent up the stack:
the FCB (frame control block), and the padding bytes inserted by
the controller into the frame payload, nor the FCS. All these are
being pulled out of the skb by gfar_process_frame().
This issue is old, likely from the driver's beginnings, however
it was amplified by recent:
commit d903ec77118c ("gianfar: simplify FCS handling and fix memory leak")
which basically added the FCS to the Rx bytecount, and so brought
this to my attention.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>