[SCSI] sd: Add error handling during flushing caches

It makes no sense to flush the cache of a device without medium.
Errors during suspend must be handled according to their causes.
Errors due to missing media or unplugged devices must be ignored.
Errors due to devices being offlined must also be ignored.
The error returns must be modified so that the generic layer
understands them.

[jejb: fix up whitespace and other formatting problems]
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>