Implement CRC16 check for L2CAP packets. FCS is used by Streaming Mode and
Enhanced Retransmission Mode and is a extra check for the packet content.

Using CRC16 is the default, L2CAP won't use FCS only when both side send
a "No FCS" request.

Initially based on a patch from Nathan Holstein <nathan@lampreynetworks.com>
Signed-off-by: NGustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>