ata: remove EXPORT_SYMBOL_GPL()s not used by modules

ata: remove EXPORT_SYMBOL_GPL()s not used by modules

Remove EXPORT_SYMBOL_GPL()s for functions used only by
the core libata code.

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

   text    data     bss     dec     hex filename
before:
  39838     573      40   40451    9e03 drivers/ata/libata-core.o
  21071     105     576   21752    54f8 drivers/ata/libata-scsi.o
  17519      18       0   17537    4481 drivers/ata/libata-eh.o
after:
  39688     573      40   40301    9d6d drivers/ata/libata-core.o
  21040     105     576   21721    54d9 drivers/ata/libata-scsi.o
  17405      18       0   17423    440f drivers/ata/libata-eh.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>