mlxsw: spectrum_nve_vxlan: Make VxLAN flags check per address family

As part of 'can_offload' checks, there is a check of VxLAN flags.

The supported flags for IPv6 VxLAN will be different from the existing
flags because of some limitations.

As preparation for IPv6 underlay support, make this check per address
family.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>