libata: fixup oops in ata_eh_link_report()

We should only try to evaluate the cdb if this is an ATAPI
device, for any other device the 'cdb' field and the cdb_len
has no meaning.

Fixes: cbba5b0e
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Tejun Heo <tj@kernel.org>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>