ath9k: Introduce bit masks for valid and valid_single_stream.

replace valid and valid_single_stream in rate table with bit masks
and reorganize the code so adding 3x3 rate control would be easier.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>