ACPI / PM: Do not execute legacy driver PM callbacks

Since all ACPI drivers in the tree should have been switched
to power management handling based on struct dev_pm_ops,
modify the ACPI bus type driver so that is doesn't execute
legacy driver power management callbacks from the functions
pointed to by the members of the acpi_bus_pm structure.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>