Keep alive being sent when PersistentKeepalive = 0

[Steve Brown]

I have no PersistentKeepalive set, but the peer gets sent a keep alive
every 60 seconds anyway.

This is verified by the both the wireguard log on the sending side and
firewall log on the receiving side. The peer is nat'd and sends its own
keep alive every 15 seconds. 

wg showconf <intf> doesn't show a keep alive value for the peer.

This isn't a problem other than a little noise in the log. The packet
gets dumped by the peer's firewall.

I'm running openwrt with kernel 5.10.23.

Steve