net/mlx5e: Don't make internal use of errno to denote missing neigh

EAGAIN is treated as a specific case when we consider the attachment
successful but wait for neigh event before offloading the flow.
This can result in unwanted behavior when sub calls on the offloading
path will return EAGAIN and we pass this error up.

Instead of attaching to a specific error code return a  boolean value
from the attach encap operation saying if the encap is valid or not.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>