net: aquantia: Remove usage of atl_fwd.private field

Remove usage of atl_fwd.private field in the IPA glue layer as the
structure member is planned for removal in future driver releases.

Change-Id: I52f73cf7dc4ee26b9e048f5360634eac96a6009b
Signed-off-by: Jinesh K. Jayakumar <jineshk@codeaurora.org>