net/mlx5: DR, Fix dumping of legacy modify_hdr in debug dump

The steering dump parser expects to see 0 as rewrite num of actions
in case pattern/args aren't supported - parsing of legacy modify header
is based on this assumption.
Fix this to align to parser's expectation.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>