arm: dma-iommu: Clean up redundant variable

mapping->size can be derived from mapping->bits << PAGE_SHIFT
which makes mapping->size as redundant.

Clean this up.

Signed-off-by: Ritesh Harjani <ritesh.harjani@gmail.com>
Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>