iommu/sun50i: Fix set-but-not-used variable warning

Fix the following warning the the SUN50I driver:

   drivers/iommu/sun50i-iommu.c: In function 'sun50i_iommu_irq':
   drivers/iommu/sun50i-iommu.c:890:14: warning: variable 'iova' set but not used [-Wunused-but-set-variable]
     890 |  phys_addr_t iova;
         |              ^~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200904113906.3906-1-joro@8bytes.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>