cnss2: Clear iova start/stop address if smmu is not enabled

The iova start and stop addresses are overwritten by MHI to
the values of DRAM start and end addresses if their values
are 0. So set the iova_start/iova_stop to 0 instead of
calling the kernel APIs memblock_start_of_DRAM() and
memblock_end_of_DRAM() when smmu_s1_enable is false. At this
point, we can set the iova start and stop address correctly
without calling the kernel APIs by the help of MHI.

CRs-Fixed: 2587029
Change-Id: I3a5bd5fcf5f66059d7e147cebc417a77f2e36d57
Signed-off-by: Guisen Yang <guiseny@codeaurora.org>