net/mlx5: Fix crash during sync firmware reset

When setting Bluefield to DPU NIC mode using mlxconfig tool +  sync
firmware reset flow, we run into scenario where the host was not
eswitch manager at the time of mlx5 driver load but becomes eswitch manager
after the sync firmware reset flow. This results in null pointer
access of mpfs structure during mac filter add. This change prevents null
pointer access but mpfs table entries will not be added.

Fixes: 5ec69744

 ("net/mlx5: Add support for devlink reload action fw activate")
Signed-off-by: Suresh Devarakonda <ramad@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Bodong Wang <bodong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://lore.kernel.org/r/20221026135153.154807-12-saeed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>