scsi: sd: Cleanup sd_done sense data handling

Use a switch for the sense key, and remove two pointless variables that
are only used once.

[mkp: Added UNMAP comment and removed good_bytes based on comment from
Damien]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>