ide: save the returned value of dma_map_sg

dma_map_sg could return a value different to 'nents' argument of
dma_map_sg so the ide stack needs to save it for the later usage
(e.g. for_each_sg).

The ide stack also needs to save the original sg_nents value for
pci_unmap_sg.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
[bart: backport to Linus' tree]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>