iommu: iommu-debug: Fix functional_fast_dma test

The __full_va_sweep() function within the functional fast DMA
test assumes that the first available mapping within the IOVA
space will be address 0. This was true when the IOVA allocation
code unconditionally set the starting search address to 0 when
it tried to wrap around to find an IOVA. However, since that is
no longer the case, change __full_va_sweep() to ensure that
the starting search address is 0 before executing the test.

Change-Id: Ib264475769fc945c1b9c74225d4189f5a2d912a6
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>