[SCSI] Reduce polling in sd.c

If a drive reports that no media is present, there's no point in
continuing to ask it about media status.  This patch (as696) cuts the
TUR polling short as soon as the drive reports no media instead of
going a full 3 iterations.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>