staging: rtl8712: fix negative level value

range->avg_qual.level is of type uint8, so it shouldn't be assigned a
negative number. Assigning it 0x100 - 78, which is the equivalent
to (20 + -98) dBm when IW_QUAL_DBM is set.
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/rtl871x_ioctl_linux.c

@@ -883,7 +883,7 @@ static int r8711_wx_get_range(struct net_device *dev,
 	range->max_qual.updated = 7; /* Updated all three */
 	range->avg_qual.qual = 92; /* > 8% missed beacons is 'bad' */
 	/* TODO: Find real 'good' to 'bad' threshold value for RSSI */
-	range->avg_qual.level = 20 + -98;
+	range->avg_qual.level = 0x100 - 78;
 	range->avg_qual.noise = 0;
 	range->avg_qual.updated = 7; /* Updated all three */
 	range->num_bitrates = RATE_COUNT;