openolt adapter crashes during OLT device deletion

During the verification of patches https://gerrit.opencord.org/c/voltha-go/+/28936 and https://gerrit.opencord.org/c/voltha-openonu-adapter-go/+/28800, a crash of the openolt adapter occurs.
The test sequence was:

• create OLT device
• enable OLT device
• restart openonu adapter
• delete OLT device

Panic seen in openolt adapter log:
{{{{

{"level":"debug","ts":"2021-11-08T10:45:23.347Z","caller":"grpc/client.go:304","msg":"endpoint-reachable","instanceId":"voltha1-voltha-adapter-openolt-5cbc48f686-vhjjf","endpoint":"voltha1-voltha-core.voltha.svc:55558"} {"level":"debug","ts":"2021-11-08T10:45:23.349Z","caller":"grpc/client.go:253","msg":"update-activity","instanceId":"voltha1-voltha-adapter-openolt-5cbc48f686-vhjjf","op-id":"4c26822eccd2aa1f","op-name":"/voltha.AdapterService/AdoptDevice","task-id":"11de4966f5ad1d1a","task-name":"interface-indication","api-endpoint":"voltha1-voltha-core.voltha.svc:55558"} {"level":"debug","ts":"2021-11-08T10:45:23.349Z","caller":"grpc/client.go:304","msg":"endpoint-reachable","instanceId":"voltha1-voltha-adapter-openolt-5cbc48f686-vhjjf","endpoint":"voltha1-voltha-core.voltha.svc:55558"}

panic: send on closed channel

goroutine 3432 [running]:
github.com/opencord/voltha-lib-go/v7/pkg/grpc.(*Client).Start.func1(0xc001252820, 0x1230780, 0xc0012457d0, 0x112dc98, 0x0, 0xc0003b3cf0, 0xc000ba69c0)
/go/src/vendor/github.com/opencord/voltha-lib-go/v7/pkg/grpc/client.go:406 +0x45b
created by github.com/opencord/voltha-lib-go/v7/pkg/grpc.(*Client).Start
/go/src/vendor/github.com/opencord/voltha-lib-go/v7/pkg/grpc/client.go:392 +0x6bd
}}}}