Bluetooth: Fix bug with ERTM minimum packet length

ERTM and streaming mode L2CAP sockets have no minimum packet length. Only
basic mode connections have minimum length.

Instead, validate the packet containing all necessary control, FCS,
and SAR fields.

The patch fixes the drop of valid packets with length lower than 4.

Signed-off-by: Nathan Holstein <ngh@isomerica.net>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>