Bluetooth: Fix authentication request for L2CAP raw sockets

When there is an existing connection l2cap_check_security needs to be
called to ensure that the security level of the new socket is fulfilled.
Normally l2cap_do_start takes care of this, but that function doesn't
get called for SOCK_RAW type sockets. This patch adds the necessary
l2cap_check_security call to the appropriate branch in l2cap_do_connect.

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>