ath9k: Enable TIM timer interrupt only when needed.

ath9k: Enable TIM timer interrupt only when needed.

commit 3f7c5c10e9dc6bf90179eb9f7c06151d508fb324 upstream.

The TIM timer interrupt is enabled even before the ACK of nullqos
is received which is unnecessary.

Also clean up the CONF_PS part of config callback properly for
better readability.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>