PCI / ACPI: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1472052 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci-acpi.c

@@ -548,6 +548,7 @@ static int acpi_pci_set_power_state(struct pci_dev *dev, pci_power_t state)
 			error = -EBUSY;
 			break;
 		}
+		/* Fall through */
 	case PCI_D0:
 	case PCI_D1:
 	case PCI_D2: