dm: ahci: Refactor to avoid static variables

With driver model we need each device to have its own state. As a step
towards this, restrict use of the global 'probe_ent' to just a few places
in the file. This will allow us to add driver-model functions which can
pass the correct data around.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>