net/mlx5e: Keep updating ethtool statistics when the interface is down

ethtool statistics should be updated even when the interface is down
since it shows more than just netdev counters, which might change while
the logical link is down.
One useful use case, for example, is when running RoCE traffic over the
interface (while the logical link is down, but physical link is up) and
examining rx_prioX_bytes.

Fixes: f62b8bb8 ("net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality")
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>