Bluetooth: Get a more accurate L2CAP PDU len

Both FCS and Extended header might be or might not be present in a ERTM
channel.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>