Bluetooth: Introduce L2CAP channel callback for suspending

Setting the BT_SK_SUSPEND socket flag from the L2CAP core is causing a
dependency on the socket. So instead of doing that, use a channel
callback into the socket handling to suspend.

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