ath9k: Fix nexttbtt calculation

nexttbtt has to be obtained from the timestamp of the beacon
obtained from mac80211. Fix this.
And <asm/unaligned.h> is not needed anymore.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>