ath9k: Parse DTIM period from mac80211

ath9k: Parse DTIM period from mac80211

With the current save power save implementation we assume a dtim period
of 1.This value is assigned based on a sanity check in the driver
eventhough we had not parsed it from mac80211.This patch obtains the actual
DTIM period from AP by parsing it from mac80211.Yet for handling
multicast traffic we may still have it as fixed rather than parsing it
from mac80211 .This does not breaks power save or anything as the sleep
duration is currently fixed in the driver.This patch may serve to improve
power save in the future by using dtim period for sleep duration and using
correct dtim period adhoc mode.

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