mac80211: make software rate control optional

Some devices implement the entire rate control in
firmware in some way, like wl1271 or like iwlwifi
which does some things in software but not a lot.
Therefore generic software rate control is rather
useless for them and just adds avoidable overhead
to the transmit path.

It's fairly simple to let drivers indicate that
they do not need rate control, but they need to
fulfil a number of conditions that we encode in
WARN_ONs.

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