net/mlx5e: Nullify table pointer when failing to create

On failing to create promisc flow steering table, the pointer is
returned with an error. Nullify it so unloading the driver won't try to
destroy a non existing table.

Failing to create promisc table may happen over BF devices when the ARM
side is going through a firmware tear down. The host side start a
reload flow. While the driver unloads, it tries to remove the promisc
table. Remove WARN in this state as it is a valid error flow.

Fixes: 1c46d740 ("net/mlx5e: Optimize promiscuous mode")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>