If the driver doesn't fill NL80211_STA_INFO_TX_BITRATE in sta_set_sinfo()
then as a fallback sta->deflink.tx_stats.last_rate is used.  Unfortunately
there's no guarantee that this has actually been set before it's used.

Originally found when 'iw <dev> link' would always return a tx rate of
6Mbps regardless of actual link speed for the QCA9337 running firmware
WLAN.TF.2.1-00021-QCARMSWP-1 in my netbook.

Use the sanity check logic from ieee80211_fill_rx_status() and refactor
that to use the new inline function.
Signed-off-by: Stephen Douthit <stephen.douthit@gmail.com>
Link: https://lore.kernel.org/r/20230213204024.3377-1-stephen.douthit@gmail.com
[change to bool ..._rate_valid() instead of int ..._rate_invalid()]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>