nl80211: fix the setting of RSSI threshold value for mesh

RSSI threshold value used for mesh peering should be in
negative value. After range checks to mesh parameters is
introduced, this is not allowed. Fix this.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>