powerpc/powernv/pci: Return failure for some uses of dma_set_mask()

Commit 8e3f1b1d

 ("powerpc/powernv/pci: Enable 64-bit devices to access
>4GB DMA space") introduced the ability for PCI device drivers to request a
DMA mask between 64 and 32 bits and actually get a mask greater than
32-bits. However currently if certain machine configuration dependent
conditions are not meet the code silently falls back to a 32-bit mask.

This makes it hard for device drivers to detect which mask they actually
got. Instead we should return an error when the request could not be
fulfilled which allows drivers to either fallback or implement other
workarounds as documented in DMA-API-HOWTO.txt.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>