brcmfmac: fix setting of station info chains bitmask

The sinfo->chains field is a bitmask for filled values in chain_signal
and chain_signal_avg, not a count. Treat it as such so that the driver
can properly report per-chain RSSI information.

Before (MIMO mode):

  $ iw dev wlan0 station dump
      ...
      signal: -51 [-51] dBm

After (MIMO mode):

  $ iw dev wlan0 station dump
      ...
      signal: -53 [-53, -54] dBm

Fixes: cae355dc ("brcmfmac: Add RSSI information to get_station.")
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210506132010.3964484-1-alsi@bang-olufsen.dk