mac80211: remove multicast check from check_tim()

Currently mac80211 checks for the multicast tim bit from beacons,
disables power save and sends a null frame if the bit is set. This was
added to support ath9k. But this is a bit controversial because the AP will
send multicast frames immediately after the beacon and the time constraints
are really high. Relying mac80211 to be fast enough here might not be
reliable in all situations. And there's no need to send a null frame, AP
will send the frames immediately after the dtim beacon no matter what.

Also if dynamic power save is disabled (iwconfig wlan0 power timeout 0)
currently mac80211 disables power save whenever the multicast bit is set
but it's never enabled again after receiving the first multicast/broadcast
frame.

The current implementation is not usable on p54/stlc45xx and the
easiest way to fix this is to remove the multicast tim bit check
altogether. Handling multicast tim bit in host is rare, most of the
designs do this in firmware/hardware, so it's better not to have it in
mac80211. It's a lot better to do this in firmware/hardware, or if
that's not possible it could be done in the driver.

Also renamed the function to ieee80211_check_tim() to follow the style
of the file.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>