ide: IDE_HFLAG_SERIALIZE_DMA bugfix

Patch re-ordering could be harmful:

commit 1fd18905
Author: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Date:   Sat Apr 26 22:25:24 2008 +0200

    ide: add IDE_HFLAG_SERIALIZE_DMA host flag
...

is buggy because ->init_dma method / ide_hwif_setup_dma() is called
before IDE_HFLAG_SERIALIZE_DMA host flag is checked.

Fix it by checking IDE_HFLAG_SERIALIZE[_DMA] after DMA initialization.

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