nfp: fix ethtool stats gather retry

The while loop fetching 64 bit ethtool statistics may have
to retry multiple times, it shouldn't modify the outside state.

Fixes: 4c352362

 ("net: add driver for Netronome NFP4000/NFP6000 NIC VFs")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>