Bluetooth: Fix suspending the L2CAP socket if we start with 0 credits

If the peer gives us zero credits in its connection request or response
we must call the suspend channel callback so the BT_SK_SUSPEND flag gets
set and user space is blocked from sending data to the socket.

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