rtl8xxxu: Feed current txrate information for mac80211

The nl80211 commands such as 'iw link' can't get current txrate
information from the driver. This commit fills in the tx rate
information from the C2H RA report in the sta_statistics function.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200320063833.1058-3-chiu@endlessm.com