scsi: sd: Quiesce warning if device does not report optimal I/O size

Commit a83da8a4 ("scsi: sd: Optimal I/O size should be a multiple
of physical block size") split one conditional into several separate
statements in an effort to provide more accurate warning messages when
a device reports a nonsensical value. However, this reorganization
accidentally dropped the precondition of the reported value being
larger than zero. This lead to a warning getting emitted on devices
that do not report an optimal I/O size at all.

Remain silent if a device does not report an optimal I/O size.

Fixes: a83da8a4

 ("scsi: sd: Optimal I/O size should be a multiple of physical block size")
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: <stable@vger.kernel.org>
Reported-by: Hussam Al-Tayeb <ht990332@gmx.com>
Tested-by: Hussam Al-Tayeb <ht990332@gmx.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>