ath9k: Use a defined value for pci_set_power_state()

Silence sparse by using a defined value PCI_D3hot instead of a magic
constant in a pci_set_power_state() call.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>