[SCSI] sr: report more accurate drive status after closing the tray.

So, what's happening here is that the drive is reporting a sense of
2/4/1 ("logical unit is becoming ready") from sr_test_unit_ready(), and
then we ask for the media event notification before checking that result
at all.  The check_media_event_descriptor() call isn't getting a check
condition, but it's also reporting that the tray is closed and that
there's no media.  In actuality it doesn't yet know if there's media or
not, but there's no way to express that in the media event status field.

My current thought is that if it told us the device isn't yet ready, we
should return that immediately, since there's nothing that'll tell us
any more data than that reliably:

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>