net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG

This reg usage is always a mapped object, not necessarily
containing chain info.

Rename to properly convey what it stores.
This patch doesn't change any functionality.

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>