ath9k: initialize tx chainmask before testing channel tx power values

With an uninitialized chainmask, the per-channel power will only contain
the power limits for a single chain instead of the combined tx power.

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