PCI / PM: Enforce type casting for pci_power_t

When casting variables of type pci_power_t, a static analysis tool complains:

  include/linux/pci.h:119:37: warning: cast from restricted pci_power_t

Enforce type casting to make the static analyzer happy.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>