ata: optimize ata_scsi_rbuf[] size

Currently the maximum required size of the ata_scsi_rbuf[] is
576 bytes in ata_scsiop_inq_89() so modify ATA_SCSI_RBUF_SIZE
define accordingly.

Code size savings on m68k arch using (modified) atari_defconfig:

   text    data     bss     dec     hex filename
before:
  20782     105    4096   24983    6197 drivers/ata/libata-scsi.o
after:
  20782     105     576   21463    53d7 drivers/ata/libata-scsi.o

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>