cdrom_id: Do not ignore errors from scsi_cmd_run()

scsi_cmd_run() can return positive error messages if we have CHECK_CONDITION
set and get the error code from the SCSI command result. So check the result
for non-zero, not for being negative.

This should fix another cause for "phantom" media in empty CD-ROM drives.

Thanks to Mike Brudevold <mike@brudevold.com> for spotting this!

https://launchpad.net/bugs/562978