LinuxKPI: 802.11: make struct ieee80211_tx_info a bit more packed

Given I was looking at the struct make it more packed at the beginning
at least. In fact it did not shrink but the tx_time_est got expanded.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 470aaf42c3c0661821a8744ae7c29b096adb4aa3)