ata: move ata_qc_complete_multiple() to libata-sata.c

* move ata_qc_complete_multiple() to libata-sata.c

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

   text    data     bss     dec     hex filename
before:
  32559     572      40   33171    8193 drivers/ata/libata-core.o
after:
  32162     572      40   32774    8006 drivers/ata/libata-core.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>