Bluetooth: Call l2cap_conn_shutdown() when SMP recv callback fails

To restore pre-l2cap_chan functionality we should be trying to
disconnect the connection when receviving garbage SMP data (i.e. when
the SMP command handler fails). This patch renames the command handler
back to smp_sig_channel() and adds a smp_recv_cb() wrapper function for
calling it. If smp_sig_channel() fails the code calls
l2cap_conn_shutdown().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>