minstrel_ht: make *idx unsigned in minstrel_downgrade_rate

net/mac80211/rc80211_minstrel_ht.c:440:46: warning: incorrect type in argument 2 (different signedness)
net/mac80211/rc80211_minstrel_ht.c:440:46:    expected int *idx
net/mac80211/rc80211_minstrel_ht.c:440:46:    got unsigned int *<noident>
net/mac80211/rc80211_minstrel_ht.c:446:46: warning: incorrect type in argument 2 (different signedness)
net/mac80211/rc80211_minstrel_ht.c:446:46:    expected int *idx
net/mac80211/rc80211_minstrel_ht.c:446:46:    got unsigned int *<noident>

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