iommu/dma: Remove unused variable

end_pfn is never used after commit <aa3ac946> ('iommu/iova: Make dma
32bit pfn implicit'), cleanup it.

Cc: Joerg Roedel <jroedel@suse.de>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>