ide: add ->max_sectors field to struct ide_port_info

* Add ->max_sectors field to struct ide_port_info to allow host drivers
  to specify value used for hwif->rqsize (if smaller than the default).

* Convert pdc202xx_old to use ->max_sectors and remove no longer needed
  IDE_HFLAG_RQSIZE_256 flag.

There should be no functional changes caused by this patch.

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