ath9k: Clarify we only want 32-bit DMA

Use DMA_32BIT_MASK to clarify we only want 32-bit DMA
memory. What was there before is also 32-bit but this makes it
clearer

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