iommu/dma: fix variable 'cookie' set but not used

The commit c1864790 ("iommu/dma: Relax locking in
iommu_dma_prepare_msi()") introduced a compliation warning,

drivers/iommu/dma-iommu.c: In function 'iommu_dma_prepare_msi':
drivers/iommu/dma-iommu.c:1206:27: warning: variable 'cookie' set but
not used [-Wunused-but-set-variable]
  struct iommu_dma_cookie *cookie;
                           ^~~~~~

Fixes: c1864790 ("iommu/dma: Relax locking in iommu_dma_prepare_msi()")
Signed-off-by: NQian Cai <cai@lca.pw>
Acked-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>

drivers/iommu/dma-iommu.c

@@ -1203,7 +1203,6 @@ int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
 {
 	struct device *dev = msi_desc_to_dev(desc);
 	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
-	struct iommu_dma_cookie *cookie;
 	struct iommu_dma_msi_page *msi_page;
 	static DEFINE_MUTEX(msi_prepare_lock); /* see below */
 
@@ -1212,8 +1211,6 @@ int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
 		return 0;
 	}
 
-	cookie = domain->iova_cookie;
-
 	/*
 	 * In fact the whole prepare operation should already be serialised by
 	 * irq_domain_mutex further up the callchain, but that's pretty subtle