ACPI / power: Remove useless message from device registering routine

After commit 71fbad6c (PCI/ACPI: Notify PCI devices when their power
resource is turned on) made acpi_pci_bind() call
acpi_power_resource_register_device(), the debug message at the end
of the latter appears in the kernel log for every PCI device that
doesn't happen to have power resources assigned (which is the vast
majority of them).  However, this message is totally useless, because
it doesn't even say which device it is about.  Moreover, it is
misleading, because it only means that the given device has no power
resources, which isn't exceptional at all.

Remove that useless message altogether and simplify
acpi_power_resource_register_device() slightly.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>