net/mlx5e: Add prefix for e-switch offloaded TC flow attributes

Add esw_ prefix to the flow attributes attached to offloaded e-switch
TC flows. This is a pre-step to add attributes to offloaded NIC TC flows.

Also, save one pointer space by using gcc's zero size array, this would
be beneficial for environments where 100Ks (or Ms) of flows are offloaded.

This patch doesn't change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>