ACPI: scan: Rename acpi_bus_get_parent() and rearrange it

The acpi_bus_get_parent() name doesn't really reflect the
purpose of the function so change it to a more accurate
acpi_find_parent_acpi_dev().

While at it, rearrange the code inside that function to make it
easier to read.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Punit Agrawal <punit.agrawal@bytedance.com>