ide: add IDE_HFLAG_SERIALIZE_DMA host flag

* Add IDE_HFLAG_SERIALIZE_DMA host flag to serialize ports
  if DMA is available and handle it in ide_init_port().

* Convert sl82c105 host driver to use this new flag.

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

include/linux/ide.h

@@ -1087,6 +1087,8 @@ enum {
 	/* unmask IRQs */
 	IDE_HFLAG_UNMASK_IRQS		= (1 << 25),
 	IDE_HFLAG_ABUSE_SET_DMA_MODE	= (1 << 26),
+	/* serialize ports if DMA is possible (for sl82c105) */
+	IDE_HFLAG_SERIALIZE_DMA		= (1 << 27),
 	/* force host out of "simplex" mode */
 	IDE_HFLAG_CLEAR_SIMPLEX		= (1 << 28),
 	/* DSC overlap is unsupported */