net/mlx4_en: Fill all counters under one call of stats lock

Before this patch, the stats_lock was acquired twice. In between the
locks Driver sent command to gather some more statistics (per priority
and counter statistics). If the stats lock was acquired by get
statistics NDO in between we would have report out of sync counters.

Fix this by collecting all stats from Firmware in advance and then
fill the Software structs under one lock.

Fixes: 0b131561

 ("net/mlx4_en: Add Flow control statistics display via ethtool")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>