staging: wfx: fix handling of frames without RSSI data

staging: wfx: fix handling of frames without RSSI data

It seems that in the old days, the RSSI information could be missing. In
this case, in order to not pollute the RSSI stats, the frame was
dropped (!).

It is far better to mark the frame with the flag RX_FLAG_NO_SIGNAL_VAL.

In add, the problem seems now fixed in the firmware (at least, it has
not been encountered with recent firmwares).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-10-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>