ath9k: make DMA memory consistent

Make the DMAable mameory consistent with pci_set_consistent_dma_mask().
The DMA-mapping.txt Documentation recommends this but for PCI-X
considerations and on strange architecture like SGI SN2, not sure
why it would fix an issue but lets see if it does, just in case.

Before this, this driver was tested with x86_64 with about
7 GB of RAM, not sure if this is really needed.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>