net/mlx4: Clean the code to eliminate trivial build warnings

Remove code that triggers trivial build warnings.

drivers/net/ethernet/mellanox/mlx4/cmd.c: In function ‘mlx4_set_vf_vlan’:
drivers/net/ethernet/mellanox/mlx4/cmd.c:2256: warning: variable ‘vf_oper’ set but not used
drivers/net/ethernet/mellanox/mlx4/mcg.c: In function ‘mlx4_map_sw_to_hw_steering_mode’:
drivers/net/ethernet/mellanox/mlx4/mcg.c:648: warning: comparison of unsigned expression < 0 is always false
drivers/net/ethernet/mellanox/mlx4/mcg.c: In function ‘mlx4_map_sw_to_hw_steering_id’:
drivers/net/ethernet/mellanox/mlx4/mcg.c:685: warning: comparison of unsigned expression < 0 is always false
drivers/net/ethernet/mellanox/mlx4/mcg.c: In function ‘mlx4_hw_rule_sz’:
drivers/net/ethernet/mellanox/mlx4/mcg.c:712: warning: comparison of unsigned expression < 0 is always false
drivers/net/ethernet/mellanox/mlx4/fw.c: In function ‘mlx4_opreq_action’:
drivers/net/ethernet/mellanox/mlx4/fw.c:1732: warning: variable ‘type_m’ set but not used
drivers/net/ethernet/mellanox/mlx4/srq.c:302: warning: no previous prototype for ‘mlx4_srq_lookup’

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>