sd: Fix missing ATO tag check

3aec2f41 introduced a merge error where we would end up check for
sdkp instead of sdkp->ATO. Fix this so we register app tag capability
correctly.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Cc: <stable@vger.kernel.org> # v3.17+
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>