Bluetooth: Create a unified auth_type evaluation function

The logic for determining the needed auth_type for an L2CAP socket is
rather complicated and has so far been duplicated in
l2cap_check_security as well as l2cap_do_connect. Additionally the
l2cap_check_security code was completely missing the handling of
SOCK_RAW type sockets. This patch creates a unified function for the
evaluation and makes l2cap_do_connect and l2cap_check_security use that
function.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>