atp870u: Improve _probe()

Move scsi_host_alloc() to the top of _probe() to remove code duplication,
*p and unneeded atpdev (de)allocation and copying.  While at it, fix the
error paths to return real error codes and also add missing
pci_disble_device() call.
Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>