policy: don't select better candidate with bad signal

policy: don't select better candidate with bad signal

Don't select a node as a better candidate where a STA has insufficient
signal to. This is the case when the roam_trigger_snr is greater than
the projected signal of the client.

This stops clients from being selected to roam to a node where the
signal is not sufficient to maintain the connection. A selection might
otherwise happen e.g. due to better channel load on the remote node.

Signed-off-by: David Bauer <mail@david-bauer.net>