ath9k: fix dma mapping leak of rx buffer upon rmmod

We were claiming DMA buffers on the RX tasklet but never
upon a simple module removal.

Cc: stable@kernel.org
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>