net/mosquitto: port is optional in root config

From mosquitto 2.x, port became optional and deprecated in the config,
and it was recommended that listeners be used instead.  Drop the hard
requirement in our config conversion script.

Reported in: https://github.com/openwrt/packages/issues/15506
Signed-off-by: <karlp@etactica.com>

net/mosquitto/files/etc/init.d/mosquitto

@@ -63,8 +63,7 @@ convert_mosq_general() {
 		echo "bind_address localhost" >> $TCONF
 	fi
 
-	config_get port "$1" port 1883
-	echo "port $port" >> $TCONF
+	append_if "$1" port
 	append_if "$1" acl_file
 	append_optional_bool "$1" allow_anonymous
 	append_optional_bool "$1" allow_duplicate_messages