policy: avoid creating kick loop for client

policy: avoid creating kick loop for client

When checking whether a client is allowed to associate to a node, the
lower ceiling for kicking clients was not taken into account when
assoc-steering is disabled.

The problem behind this is, that a configured lower barrier for
disassociating clients (kicking) would kick the client immediatly after
association. In the worst scenario the client immediatly associates
again to the station and ends up in a kick loop.

Don't allow associating when a min_snr is configured and the client
signal is below this value.

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