net/mlx5e: Move code chunk setting encap dests into its own function

Split setting encap dests code chunk out of mlx5e_tc_add_fdb_flow()
to make the function smaller for maintainability and reuse.
For symmetry do the same for mlx5e_tc_del_fdb_flow().
While at it refactor cleanup to first check for encap flag like
done when setting encap dests.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>