tests/afr: select correct read-child for data OPs.

Change-Id: If84bc489b6c45bde3bdb858da5f1600cea78c8a5
BUG: 1080759
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/7345
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>