iommu/sun50i: Remove unused variable

The pte_dma variable in the unmap callback is set but never used. Remove
it.

Fixes: 4100b8c2

 ("iommu: Add Allwinner H6 IOMMU driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200628180844.79205-2-maxime@cerno.tech
Signed-off-by: Joerg Roedel <jroedel@suse.de>