mac80211: remove sta TIM flag, fix expiry TIM handling

The TIM flag that is kept in each station's info is completely
useless, there's no code (aside from the debugfs display code)
checking it, hence it can be removed. While doing that, I noticed
that the TIM handling is broken when buffered frames expire, so
fix that.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>