cfg80211: fix reporting 64-bit station info tx bytes

Copy & paste mistake - STATION_INFO_TX_BYTES64 is the name of the flag,
not NL80211_STA_INFO_TX_BYTES64.

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