au1xxx-ide: au_ide_probe() fixes

* hwif->hold is always set in au_ide_probe() (few lines earlier),
  no need to set it again

* hwif->{channel,select_data,config_data} should be always zero

* autotune PIO also if CONFIG_BLK_DEV_IDE_AU1XXX_MDMA2_DBDMA is defined

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