uboot-imx

dm: ahci: Correct uclass private data

This is expected to be attached to the uclass and the code operates that
way, but the uclass has not been updated. Fix it to avoid using memory at
address 0.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 47fc61a (dm: ahci: Drop use of probe_ent)