ath9k: Enable TIM timer interrupt only when needed.

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>