PNP: make resource assignment functions return 0 (success) or -EBUSY (failure)

This patch doesn't change any behavior; it just makes the return
values more conventional.

This changes pnp_assign_dma() from a void function to one that
returns an int, just like the other assignment functions.  For
now, at least, pnp_assign_dma() always returns 0 (success), so
it appears to never fail, just like before.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>