mac80211: minstrel_ht: fix default max throughput rate indexes

Use the first supported rate instead of 0 (which can be invalid)

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20190820095449.45255-3-nbd@nbd.name


Signed-off-by: Johannes Berg <johannes.berg@intel.com>