Bluetooth: check for l2cap header in start fragment

BLUETOOTH SPECIFICATION Version 4.0 [Vol 3] page 36 mentioned
"Note: Start Fragments always begin with the Basic L2CAP header
of a PDU."

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@nokia.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>