net/mlx4_en: Do not query stats when device port is down

There are no counters allocated to the eth device when the port is down, so
this query is meaningless at that time.

It also leads to querying incorrect counters (since the counter_index is not
valid when the device port is down).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>