mac80211_hwsim: report fixed signal strength

There's no reason to think that hwsim has any
actual signal strength, but for testing it is
very useful to have it report _some_ value to
the stack so I can see if the value ends up
being reported correctly

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>