pnp: restore automatic resolution of DMA conflicts

To fix a 5-year-old regression, reverse changes made by commit
7ef3639 (PNP: don't fail device init if no DMA channel available).

As an example to show the problem, my sound card provides a
prioritized list of PnP "dependent sets" of requested resources:

dependent set 0 (preferred) wants DMA 5.
dependent set 1 (acceptable) will take DMA 5, 6, or 7.
...
dependent set 4 (acceptable) doesn't request a high DMA.

If DMA 5 is not available, pnp_assign_dma has to fail on set 0 so that
pnp_auto_config_dev will move on to set 1 and get DMA 6 or 7.
Instead, pnp_assign_dma adds the resource with flags |=
IORESOURCE_DISABLED and returns success. pnp_auto_config_dev just
sees success and therefore chooses set 0 with a disabled DMA and never
tries the sets that would have resolved the conflict.

Furthermore, this mode of "success" is unexpected and unhandled in
sound/isa/sb and probably other drivers. sb assumes that the returned
DMA is enabled and obliviously uses the invalid DMA number. Observed
consequences were sb successfully grabbing a DMA that was expressly
forbidden by the kernel parameter pnp_reserve_dma.

The only upside to the original change would be as a kludge for
devices that can operate in degraded mode without a DMA but that don't
provide the corresponding non-preferred dependent set. The right
workaround for those devices is to synthesize the missing set in
quirks.c; otherwise, you're reinventing PnP fallback functionality at
the driver level for that device and all others.

Signed-off-by: David Flater <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/pnp/manager.c

···
 	res->start = -1;
 	res->end = -1;
 
+	if (!rule->map) {
+		res->flags |= IORESOURCE_DISABLED;
+		pnp_dbg(&dev->dev, "  dma %d disabled\n", idx);
+		goto __add;
+	}
+
 	for (i = 0; i < 8; i++) {
 		if (rule->map & (1 << xtab[i])) {
 			res->start = res->end = xtab[i];
···
 				goto __add;
 		}
 	}
-#ifdef MAX_DMA_CHANNELS
-	res->start = res->end = MAX_DMA_CHANNELS;
-#endif
-	res->flags |= IORESOURCE_DISABLED;
-	pnp_dbg(&dev->dev, "  disable dma %d\n", idx);
+
+	pnp_dbg(&dev->dev, "  couldn't assign dma %d\n", idx);
+	return -EBUSY;
 
 __add:
 	pnp_add_dma_resource(dev, res->start, res->flags);