ath9k: Cleanup buffer status handling

Using a u32 to store a single flag is overkill.
Use a bool to store whether the buffer is stale or not.
Also, use u8 instead of u32 to store the buffer type.

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