net/mlx4: Use Kconfig flag to remove support of old gen2 Mellanox devices

Since Mellanox focus is on newer adapters, we would like to have the
ability to disable the support for old gen2 adapters.

This can be done by turning off the MLX4_CORE_GEN2 Kconfig flag.
We keep it turned on by default.

Signed-off-by: Slava Shwartsman <slavash@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>