iwlwifi: select proper rate control algorithm

Prior to this patch, iwlwifi would always use the first
registered rate control algorithm which, depending on system
setup, could be anything. After the mac80211 patch to make
the simple algorithm built-in, it would always be simple.

This has always been a bug in iwlwifi.

This fixes it by requesting that mac80211 selects the right
rate control algorithm.

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