net/mlx5: E-Switch, Use same source for offloaded actions check

Align the checks for modify header and encap actions with the
rest of the code.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>