net/mlx5: LAG, check if all eswitches are paired for shared FDB

Shared FDB LAG can only work if all eswitches are paired.
Also, whenever two eswitches are paired, devcom is marked as ready.

Therefore, in case of device with two eswitches, checking devcom was
sufficient. However, this is not correct for device with more than
two eswitches, which will be introduced in downstream patch.
Hence, check all eswitches are paired explicitly.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>