[SCSI] scsi_proc.c: display sdev->scsi_level correctly

This patch (as833) fixes the "SCSI revision" output for
/proc/scsi/scsi.  If the scsi_level value is 0 (UNKNOWN), we want it
to show up as "0", not "ffffffff".

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