Bluetooth: Mesh: Transport: Fix calculating SeqAuth

This patch fixes case where Mesh is receiving segmented data with
SEQ and SeqZero as in logs below:

bt_mesh_trans_recv: src 0x07FF dst 0x0001 seq 0x00031FFF friend_match 0
bt_mesh_trans_recv: Payload dd7ffc016e5c7f861272e5f7577a6644
trans_seg: ASZMIC 0 AKF 1 AID 0x1D
trans_seg: SeqZero 0x1FFF SegO 0 SegN 1
trans_seg: ######## seq_auth=204799

bt_mesh_trans_recv: src 0x07FF dst 0x0001 seq 0x00032000 friend_match 0
bt_mesh_trans_recv: Payload dd7ffc21abb5a7
trans_seg: ASZMIC 0 AKF 1 AID 0x1D
trans_seg: SeqZero 0x1FFF SegO 1 SegN 1
trans_seg: ######## seq_auth=212991

Original-patch-by: Łukasz Rymanowski <lukasz.rymanowski@codecoup.pl>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>