Bluetooth: Check the minimum {I,S}-frame size into L2CAP

All packets with size fewer than the minimum specified is dropped.
Note that the size of the l2cap basic header, FCS and SAR fields are
already subtracted of len at the moment of the size check.

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>