net/mlx4_core: Replace sscanf() with kstrtoint()

It is not safe to use sscanf.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.com>
Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>