remove pci_dma_* abuses and workarounds

[Christoph Hellwig]

Back before the dawn of time pci_dma_* with a NULL pci_dev argument
was used for all kinds of things, e.g. dma mapping for non-PCI
devices.  All this has been long removed, but it turns out we
still care for a NULL pci_dev in the wrappers, and we still have
two odd USB drivers that use pci_dma_alloc_consistent for allocating
memory while ignoring the dma_addr_t entirely.

This series switches the two drivers to use plain kzalloc and then
removes the handling of the NULL pci_dev in the pci_dma_* wrappers.