ath9k: Fix bug in deciphering channel flags

CHANNEL_CCK flag is set for all 2 Ghz channels, so IS_CHAN_CCK() would
turn out to be true for all channles in that band.
Use IS_CHAN_B() now, which checks the channel mode and not the channel
flags.

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