Bluetooth: Mesh: Transport seg with few adv bufs

Fixes bug where the transport segmented tx would decrement the attempt
counter every time it ran out of buffers. If transport ran out of
buffers 4 times before the sending could complete, the transfer would
end prematurely.

Moves the attempt decrement to only execute when all segments have been
sent.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>