iommu: Modify the description of iommu_sva_unbind_device

iommu_sva_unbind_device has no return value.

Remove the description of the return value of the function.

Signed-off-by: Chen Jun <c00424029@huawei.com>
Link: https://lore.kernel.org/r/20201023064827.74794-1-chenjun102@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>