dmatest: fix build warning on mips

drivers/dma/dmatest.c:543:11: warning: passing argument 1 of 'virt_to_phys' makes pointer from integer without a cast [enabled by default]

mips expects virt_to_phys() to take a pointer.  Fix up the types accordingly.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>