net/mlx5e: Move the encap entry structure from the eswitch header

The encap entry structure isn't manipulated by the eswitch code,
hence it can/needs to be removed from the eswitch header.

Do that, and change it to have mlx5e_ prefix.

This patch doesn't change any functionality.

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