PCI/PM: Clean up pci_set_low_power_state()

Make the following assorted non-essential changes in
pci_set_low_power_state():

 1. Drop two redundant checks from it (the caller takes care of these
    conditions).

 2. Change the log level of a messages printed by it to "debug",
    because it only indicates a programming mistake.

Link: https://lore.kernel.org/r/2539071.Lt9SDvczpP@kreacher


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>