After rework settings will be no longer created dynamically
for each device so we need to make some fixups first.

* Use set_[ksettings,unmaskirq]() as a set function for
  ["keepsettings","unmaskirq"] setting.

* Allow writes to ["io_32bit","unmaskirq"] settings also when
  drive->no_[io_32bit,unmask] is set (this is checked later inside
  set_[io_32bit,unmaskirq]() anywyay and keeps consistency with
  the corresponding HDIO_SET_[32BIT,UNMASKINTR] ioctls).

* Use max possible multi sectors value (16) as an allowed max for
  "multcount" setting.  set_multcount() set function checks against
  device's max possbile value anyway and it makes the proc setting
  consistent with the corresponding HDIO_SET_MULTCOUNT ioctl.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>