mac80211: minstrel_ht: make att_hist and succ_hist u32 instead of u64

They are only used for debugging purposes and take a very long time to
overflow. Visibly reduces the size of the per-sta rate control data.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>