mac80211: better rate control algorithm selection

This patch changes mac80211's Kconfig/Makefile to:
 * select between the PID and the SIMPLE rate control
   algorithm as default
 * always allow tri-state for the rate control algorithms,
   building those that are selected 'y' into the mac80211
   module (if that is a module, otherwise all into the kernel)
 * force the default rate control algorithm to be built into
   mac80211

It also makes both rate control algorithms proper modules again
with MODULE_LICENSE etc.

Only if EMBEDDED is the user allowed to select "NONE" as default
which will cause no algorithm to be selected, this will work
only when the driver brings one itself (e.g. iwlwifi drivers).

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