net/mlx5e: Allow association of a flow to multiple encaps

Currently a flow can be associated with a single encap entry. The
extended destination feature enables the driver to configure multiple
encap entries per flow.

Change the encap flow association field to array as a pre-step towards
supporting multiple encap destinations. Use only the first array
element, with no functional change.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>