iommu/sun50i: Fix compile warnings

A few compile warnings show up when building this driver:

  CC      drivers/iommu/sun50i-iommu.o
drivers/iommu/sun50i-iommu.c: In function ‘sun50i_dte_get_page_table’:
drivers/iommu/sun50i-iommu.c:486:16: warning: unused variable ‘flags’ [-Wunused-variable]
  486 |  unsigned long flags;
      |                ^~~~~
drivers/iommu/sun50i-iommu.c: In function ‘sun50i_iommu_unmap’:
drivers/iommu/sun50i-iommu.c:559:23: warning: unused variable ‘iommu’ [-Wunused-variable]
  559 |  struct sun50i_iommu *iommu = sun50i_domain->iommu;
      |                       ^~~~~
drivers/iommu/sun50i-iommu.c: In function ‘sun50i_iommu_probe_device’:
drivers/iommu/sun50i-iommu.c:749:22: warning: unused variable ‘group’ [-Wunused-variable]
  749 |  struct iommu_group *group;
      |                      ^~~~~

Remove the unused variables.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200514124621.25999-1-joro@8bytes.org