mac80211: allow easier multicast/broadcast buffering in hardware

There are various decisions influencing the decision whether to buffer
a frame for after the next DTIM beacon. The "do we have stations in PS
mode" condition cannot be tested by the driver so mac80211 has to do
that. To ease driver writing for hardware that can buffer frames until
after the next DTIM beacon, introduce a new txctl flag telling the
driver to buffer a specific frame.

While at it, restructure and comment the code for multicast buffering
and remove spurious "inline" directives.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>