wifi: brcmfmac: support CQM RSSI notification with older firmware

Using the BCM4339 firmware from linux-firmware (version "BCM4339/2 wl0:
Sep  5 2019 11:05:52 version 6.37.39.113 (r722271 CY)" from
cypress/cyfmac4339-sdio.bin) the RSSI respose is only 4 bytes, which
results in an error being logged.

It seems that older devices send only the RSSI field and neither SNR nor
noise is included.  Handle this by accepting a 4 byte message and
reading only the RSSI from it.

Fixes: 7dd56ea4 ("brcmfmac: add support for CQM RSSI notifications")
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230124104248.2917465-1-john@metanate.com