ath9k: set ATH9K_PCI to y by default

Most ath9k devices are PCI/PCIe based, therefor making PCI/PCIe support
default y helps those porting a config from 2.6 kernel series
from getting "non-functional" wireless drivers with 3.x kernel series.

Signed-off-by: Daniel Kuehn <daniel@kuehn.se>
Signed-off-by: John W. Linville <linville@tuxdriver.com>