powerpc/pseries: Do not need to check for dma_window == NULL

The block in pci_dma_dev_setup_pSeriesLP for dma_window == NULL can be
removed because we will only teminate the loop if we had already allocated
a iommu table for that node or we found a window.  While there may be
no window for the device, the intresting part is if we are reusing a
table or creating it for the first device under it.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>