net/mlx5e: Always clear dest encap in neigh-update-del

The cited commit introduced a bug for multiple encapsulations flow.
If one dest encap becomes invalid, the flow is set slow path flag.
But when other dests encap become invalid, they are not cleared due
to slow path flag of the flow. When neigh-update-add is running, it
will use invalid encap.

Fix it by checking slow path flag after clearing dest encap.

Fixes: 9a5f9cc7

 ("net/mlx5e: Fix possible use-after-free deleting fdb rule")
Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>