sd: fix an error return in probe()

If device_add() fails then it should return the error code but instead
the current code returns success.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>