ath9k: Synchronize DMA transfer with CPU at right place

This patch does pci_dma_sync_single_for_cpu() before accessing
the header of the frame and queueing the same buffer into h/w.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>