ide: move ide_map_sg() call out of ->dma_setup method (take 2)

Move ide_map_sg() call from ->dma_setup implementations and
ide_destroy_dmatable() one from *_build_dmatable() to ide_dma_prepare().

There should be no functional changes caused by this patch.

Sergei:
Removed 'use_pio_instead' labels and replaced 'goto' with 'return 0' --
that required no changes to the follow-up patches...

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>