sd: Optimal I/O size is in bytes, not sectors

Commit ca369d51 ("block/sd: Fix device-imposed transfer length
limits") accidentally switched optimal I/O size reporting from bytes to
block layer sectors.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Fixes: ca369d51
Cc: stable@vger.kernel.org # 4.4+
Reviewed-by: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>