net/mlx5e: Fix error path of device attach

On failure to attach the netdev, fix the rollback by re-setting the
device's state back to MLX5E_STATE_DESTROYING.

Failing to attach doesn't stop statistics polling via .ndo_get_stats64.
In this case, although the device is not attached, it falsely continues
to query the firmware for counters. Setting the device's state back to
MLX5E_STATE_DESTROYING prevents the firmware counters query.

Fixes: 26e59d80

 ("net/mlx5e: Implement mlx5e interface attach/detach callbacks")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>