wifi: rtl8xxxu: Fix the channel width reporting

The gen 2 chips RTL8192EU and RTL8188FU periodically send the driver
reports about the TX rate, and the driver passes these reports to
sta_statistics. The reports from RTL8192EU may or may not include the
channel width. The reports from RTL8188FU do not include it.

Only access the c2h->ra_report.bw field if the report (skb) is big
enough.

The other problem fixed here is that the code was actually never
changing the channel width initially reported by
rtl8xxxu_bss_info_changed because the value of RATE_INFO_BW_20 is 0.

Fixes: 0985d3a4 ("rtl8xxxu: Feed current txrate information for mac80211")
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/5b41f1ae-72e7-6b7a-2459-b736399a1c40@gmail.com