net: gianfar: Convert to ndo_get_stats64 interface

No reason to produce the legacy net_device_stats struct, only to have it
converted to rtnl_link_stats64.  And as a bonus, this allows for improving
counter size to 64 bit.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>