Bluetooth: Mesh: Add missing adv_main for relay message

By design, adv main set can use to send relay mesh messages
but here adv main set is invoked.

Add in the ended, so, adv main set only can be used when all
relay adv sets busy.

and this way does not cause too much delay for adv main set,
due to local message will process firstly.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>