ath9k: Fix bug in TX DMA termination

Removing the module was slow because ath9k_hw_stopdma()
was looping for a long time quantum. Use reasonable
values now to fix this issue.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>