msm_geni_serial: Use the common driver API for dma_alloc

Use the common driver APIs instead of directly using
dma_alloc_coherent,dma_free_coherent APIs.
Issue is that we need to pass iommu dev as a handler
while using dma APIs where as we were passing common
driver dev node as a handler which is not correct.

Change-Id: Ia8da31f8b9d761a723b3712c7b0738e71838fa56
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>