swiotlb will only bounce buffer when the effective dma address for the
device is smaller than the actual DMA range.  Instead of flipping between
the swiotlb and nommu ops for FSL SOCs that have the second outbound
window just don't set the bus dma_mask in this case.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Scott Wood <oss@buserror.net>