brcmfmac: add fallback for devices that do not report per-chain values

If brcmf_cfg80211_get_station fails to determine the RSSI from the
per-chain values get the value individually as a fallback.

Fixes: 1f0dc59a

 ("brcmfmac: rework .get_station() callback")
Signed-off-by: Jaap Jan Meijer <jjmeijer88@gmail.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>