net/mlx4_en: Protect access to the statistics bitmap

This will allow parallel access to the statistics bitmap.
A pre-step for adding PFC counters, where the statistics bitmap
can be dynamically changed when modifying the PFC setting.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>