Bluetooth: L2CAP: Fix accounting SDU header len as data

This cause packets that are up to 2 bytes off segment maximum length
to be considered transmitted when in fact that could be some bytes left
to be transmitted which cause any subsequent packet to trigger invalid
SDU on the remote end:

[ 3612.376068] l2cap_le_data_rcv:6757: SDU fragment. chan->sdu->len 66 skb->len 68 chan->sdu_len 67
[ 3612.376073] Bluetooth: Too much LE L2CAP data received

Change-Id: Id2f3469ce1c0b27bb87c4d5bc18e6ede9d93dbde
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>