Bluetooth: Mesh: Transport tx fields overflow

The transport segmented TX nack and seg_pending fields must be at least
6 bits to avoid overflow for 32 segment messages. This change rearranges
the seg_tx fields to gather all state flag fields in one byte, while
making the counter fields whole bytes.

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