ath9k: disable RNG by default

ath9k RNG will dominates all the noise sources from the real HW
RNG, disable it by default. But we strongly recommand to enable
it if the system without HW RNG, especially on embedded systems.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Acked-by: Stephan Mueller <smueller@chronox.de>
Acked-by: Stephan Mueller <smueller@chronox.de>
Reviewed-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>