platform/x86: dell_rbu: stop abusing the DMA API

For some odd reason dell_rbu actually seems to want the physical and
not a bus address for the allocated buffer.  Lets assume that actually
is correct given that it is BIOS-related and that is a good source
of insanity.  In that case we should not use dma_alloc_coherent with
a NULL device to allocate memory, but use GFP_DMA32 to stay under
the 32-bit BIOS limit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>