net/mlx5: E-Switch, Add operational mode to the SRIOV e-Switch

Define three modes for the SRIOV e-switch operation, none (SRIOV_NONE,
none of the VF vports are enabled), legacy (SRIOV_LEGACY, the current mode)
and sriov offloads (SRIOV_OFFLOADS). Currently, when in SRIOV, only the
legacy mode is supported, where steering rules are of the form:

        destination mac --> VF vport

This patch does not change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>