We can only handle BARs above 4GB if dma_addr_t (not resource_size_t) is 64
bits wide.  If we have a 64-bit resource_size_t and a 32-bit dma_addr_t,
we can't deal with BARs above 4GB.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>