staging: vt6656: fix sign of rx_dbm to bb_pre_ed_rssi.

BugLink: https://bugs.launchpad.net/bugs/1868627

commit 93134df5 upstream.

bb_pre_ed_rssi is an u8 rx_dm always returns negative signed
values add minus operator to always yield positive.

fixes issue where rx sensitivity is always set to maximum because
the unsigned numbers were always greater then 100.

Fixes: 63b9907f ("staging: vt6656: mac80211 conversion: create rx function.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/aceac98c-6e69-3ce1-dfec-2bf27b980221@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

drivers/staging/vt6656/dpc.c

@@ -144,7 +144,7 @@ int vnt_rx_data(struct vnt_private *priv, struct vnt_rcb *ptr_rcb,
 
 	vnt_rf_rssi_to_dbm(priv, *rssi, &rx_dbm);
 
-	priv->bb_pre_ed_rssi = (u8)rx_dbm + 1;
+	priv->bb_pre_ed_rssi = (u8)-rx_dbm + 1;
 	priv->current_rssi = priv->bb_pre_ed_rssi;
 
 	frame = skb_data + 8;