net/mlx5: Devcom, serialize devcom registration

From one hand, mlx5 driver is allowing to probe PFs in parallel.
From the other hand, devcom, which is a share resource between PFs, is
registered without any lock. This might resulted in memory problems.

Hence, use the global mlx5_dev_list_lock in order to serialize devcom
registration.

Fixes: fadd59fc ("net/mlx5: Introduce inter-device communication mechanism")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>