net/mlx5: EQ, Remove redundant completion EQ list lock

Completion EQs list is only modified on driver load/unload, locking is
not required, remove it.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>