Fixed a bug in the cache module of RE

Fixed a bug in the cache module of RE

If observation for a resource from a client is started, it should be canceled.
But there was a logic error that it isn't.

Change-Id: I2440c20999b7076bea5987ccbad3f0710269b3d9
Signed-off-by: coderhyme <jhyo.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5807
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: JungYong KIM <jyong2.kim@samsung.com>
(cherry picked from commit 607ef0e262f85d164cb3b1edbcf2e5bf926a2930)
Reviewed-on: https://gerrit.iotivity.org/gerrit/6259