net: remove the PCI_DMA_BUS_IS_PHYS check in illegal_highdma

These days the dma mapping routines must be able to handle any address
supported by the device, be that using an iommu, or swiotlb if none is
supported.  With that the PCI_DMA_BUS_IS_PHYS check in illegal_highdma
is not needed and can be removed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>