mac80211: Fix rate reporting regression

Mattias Nissler's "clean up rate selection" patch incorrectly changes
the behavior of txrate setting in sta_info. This patch backs out parts
of the rate selection consolidation in order to fix this issue for now.

Signed-off-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>