ath9k_hw: remove IS_CHAN_B()

Hardware 802.11b-only mode isn't supported by the driver (the device is
configured for 802.11n/g instead). Simplify the code by removing checks
for it.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>