net/mlx5: allow modify field action on group 0

net/mlx5: allow modify field action on group 0

There is a limitation about copying one header field to another for
the Flow group 0. Such copy action is not allowed there. But setting
a header field with an immediate value is perfectly fine.
Allow the MODIFY_FIELD action on group 0 in case the source field
is an immediate value or a pointer to it.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>