ata: simplify ata_scsiop_inq_89()

Initialize rbuf[] directly instead of using ata_tf_to_fis(). This
results in simpler and smaller code. It also allows separating
ata_tf_to_fis() into SATA specific libata part in the future.

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

   text    data     bss     dec     hex filename
before:
  20824     105    4096   25025    61c1 drivers/ata/libata-scsi.o
after:
  20782     105    4096   24983    6197 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>