ath9k: use pci_dev->subsystem_device

The driver reads PCI subsystem ID from the PCI configuration register while it's
already stored by the PCI subsystem in the 'subsystem_device' field of 'struct
pci_dev'...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>