Bluetooth: Mesh: Transport tx seg_o overflow

Increases the transport segmentented tx seg_o counter to 6 bits to avoid
overflow when sending 32 segments. The check in the send loop would
previously never be false, which causes segments to repeat
unnecessarily.

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