Move common/utils.go to device/utils.go.

The utility functions ConvB2S(), OnuToSn(), LoggerWithOnu() in common/utils.go strongly depend on ONU instance specification.
Then, I consider it should be in device package that contains ONU instance , not utils package.

Change-Id: I7fd2b21c5f7c51be8ec4a5c161da9c93e3a3f582
diff --git a/core/omci.go b/core/omci.go
index a83c07a..2566f2a 100644
--- a/core/omci.go
+++ b/core/omci.go
@@ -18,6 +18,7 @@
 
 import (
 	"context"
+
 	"gerrit.opencord.org/voltha-bbsim/common/logger"
 	openolt "gerrit.opencord.org/voltha-bbsim/protos"
 	omci "github.com/opencord/omci-sim"
@@ -43,7 +44,6 @@
 					omciIn <- resp
 					s.handleOmciAction(resp.Pkt, resp.IntfId, resp.OnuId)
 
-
 				case *omci.OmciError:
 					// Error in processing omci message. Log and carry on.
 					logger.Debug("%s", err.Msg)