ath9k: add a debugfs entry for ignoring CCA on the extension channel in HT40

Debugfs requires a u32 for bool knobs though so we turn the
ath9k_hw knob into a u32 as well.

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