Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

Cc: Len Brown <lenb@kernel.org>
Cc: linux-acpi@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20230322083646.2937580-1-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>