libata: reorder ata_device to remove 8 bytes of padding on 64 bits

reduce size by 8 bytes from 1160 to 1152 allowing it to fit in 1 fewer
cachelines.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>