When doing dma_alloc_coherent in the driver, we add a certain hard-coded
offset to the DMA address before returning to the callee function. This
offset is needed when our device use this DMA address to perform
outbound transactions to the host.

However, if we want to map the DMA'able memory to the user via
dma_mmap_coherent(), we need to pass the original dma address, without
this offset. Otherwise, we will get erronouos mapping.
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>