ata: make "libata.force" kernel parameter optional

Add ATA_FORCE config option (visible only if EXPERT config
option is enabled) and make "libata.force" kernel parameter
optional.

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

   text    data     bss     dec     hex filename
w/ CONFIG_ATA_FORCE=y:
  31983     572      40   32595    7f53 drivers/ata/libata-core.o
w/ CONFIG_ATA_FROCE=n:
  28958     316      32   29306    727a drivers/ata/libata-core.o

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