Bluetooth: Introduce L2CAP channel callback for resuming

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

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