mac80211 rate control: fix section mismatch

When the rate control algorithms are built-in, their exit
functions can be called from mac80211's init function so
they cannot be marked __exit.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>