policy: add min_snr_kick_delay

policy: add min_snr_kick_delay

Currently the min_snr option will result in kicking clients the first
time their SNR dips below.

This might not be desirable, as drivers (notably ath10k) sometimes
report a much lower RSSI for a short timeframe after returning to
sensible values. Also, a device might be in the process of roaming just
to be kicked before.

Add the min_snr_kick_delay option. A client will be kicked after this
timeframe when it's SNR is below the min_snr threshold value over the
complete timeframe.

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