Create the float property if it doesn't exist.

If we don't have the float property we won't be able to use float properties
for device configuration since the drivers may not understand it.
We might still want to apply properties for client settings though.

Reported-by: Simon Thum <simon.thum@gmx.de>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>