Bluetooth: Mesh: Use a single transmission from Friend to LPN

As per TSE 10009 the TS will enforce that a Friend ever only uses a
single transmission when sending packets to an LPN. Make sure that our
implementation follows this.

https://www.bluetooth.org/tse/errata_view.cfm?errata_id=10009



Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>