IB/ipoib: Let lower driver handle get_stats64 call

The driver checks if the lower level driver supports get_stats, and if
so calls it to get the updated statistics, otherwise takes from the
current netdevice stats object.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>