mac80211: Check rate->idx before rate->count

The drivers are not required to fill in rate->count if rate->idx is set
to -1. Hence, we should first check rate->idx before accessing
rate->count.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>