mac80211_hwsim: Incorporate txpower into rssi

Up to now mac80211_hwsim has been reporting an rssi of -50. This patch
improves the model slightly by returning txpower-50. This makes it
easy to stimulate tests that need to see a varying rssi.

Signed-off-by: Blaise Gassend <blaise@willowgarage.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>