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 <dave@flaterco.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>