Bluetooth: Check if SDU size is greater than MTU on L2CAP

After reassembly the SDU we need to check his size. It can't overflow
the MTU size.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>