minstrel_ht: fix check for downgrading of top2 rate

The check should be against current top2 rate, instead of
current top rate.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>