net/mlx5: devcom only supports 2 ports

Devcom API is intended to be used between 2 devices only add this
implied assumption into the code and check when it's no true.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>