powerpc/powernv: Update dev->dma_mask in pci_set_dma_mask() path

powerpc defines various machine-specific routines for handling
pci_set_dma_mask().  The routines for machine "PowerNV" may neglect
to set dev->dma_mask.  This could confuse anyone (e.g. drivers) that
consult dev->dma_mask to find the current mask.  Set the dma_mask in
the PowerNV leaf routine.

Signed-off-by: Brian W. Hart <hartb@linux.vnet.ibm.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>