[SCSI] sr: update to follow tray status correctly

Based on an original patch from: David Martin <tasio@tasio.net>

When trying to get the drive status via ioctl CDROM_DRIVE_STATUS, with
no disk it gives CDS_TRAY_OPEN even if the tray is closed.

ioctl works as expected with ide-cd driver.

Gentoo bug report: http://bugs.gentoo.org/show_bug.cgi?id=196879

Cc: Maarten Bressers <mbres@gentoo.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>