iommu_sva_unbind_device has no return value.

Remove the description of the return value of the function.
Signed-off-by: NChen Jun <c00424029@huawei.com>
Link: https://lore.kernel.org/r/20201023064827.74794-1-chenjun102@huawei.comSigned-off-by: NWill Deacon <will@kernel.org>