[IOT-1775] Fix observer response handle logic

[IOT-1775] Fix observer response handle logic

Fix observer response handle logic to return OC_STACK_OK,
cases of observer request fail and observer cancel response
will handled with this logic.
[As is]
return OC_STACK_UNAUTHORIZED_REQ
[To be]
return OC_STACK_OK

Change-Id: I7e36775b71d6e4baa3f611b27c2e1cec46779b66
Signed-off-by: bg.chun <bg.chun@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/16999
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>