ath9k: reduce listen time period

When we have downlink traffic alone and the station is going thru
bgscan, the client is out of operating channel for around 1000ms which
is too long. The mac80211 decides when to switch back to oper channel
based on tx queue, bad latency and listen time. As the station does not
have tx traffic, the bgscan can easily affect downlink throughput. By
reducing the listen time, it helps the associated AP to retain the
downstream rate.

Cc: Paul Stewart <pstew@google.com>
Tested-by: Gary Morain <gmorain@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>