net/mlx5: DR, Fix uninitialized var warning

Smatch warns this:

drivers/net/ethernet/mellanox/mlx5/core/steering/dr_table.c:81
 mlx5dr_table_set_miss_action() error: uninitialized symbol 'ret'.

Initializing ret with -EOPNOTSUPP and fix missing action case.

Fixes: 7838e172 ("net/mlx5: DR, Expose steering table functionality")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>