net/mlx5e: Fix possible modify header actions memory leak

The cited commit could disable the modify header flag, but did not free
the allocated memory for the modify header actions. Fix it.

Fixes: 27c11b6b ("net/mlx5e: Do not rewrite fields with the same match")
Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>