ath9k_htc: Do not support PowerSave by default

It is a copy/paste of patch provided by Sujith for ath9k.

"Even though we make sure PowerSave is not enabled by default
by disabling the flag, WIPHY_FLAG_PS_ON_BY_DEFAULT on init,
PS could be enabled by userspace based on various factors
like battery usage etc. Since PS in ath9k is just broken
and has been untested for years, remove support for it, but
allow a user to explicitly enable it using a module parameter."

Cc: stable@vger.kernel.org
Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>