ath9k: Make PM-QOS value as user configurable

This patch allows the pm-qos value to be user configurable by making it as
a module parameter.This will help our customers to configure the pm-qos
value according to the effect in throughput due to the DMA latency problem
which was observed in Intel Pinetrail platforms.
	The tested value of '55' will be filled as the default
pm-qos-value incase the user does not specifies pm-qos value as a
module parameter.
example usage: sudo modprobe ath9k pmqos=65

Cc: Senthilkumar Balasubramanian <Senthilkumar.Balasubramanian@Atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>