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: Qian Cai <cai@lca.pw>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>