ide: keep track of number of bytes instead of sectors in struct ide_cmd

* Pass number of bytes instead of sectors to ide_init_sg_cmd().

* Pass number of bytes to process to ide_pio_sector() and rename
  it to ide_pio_bytes().

* Rename ->nsect field to ->nbytes in struct ide_cmd and use
  ->nbytes, ->nleft and ->cursg_ofs to keep track of number of
  bytes instead of sectors.

There should be no functional changes caused by this patch.

Acked-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>