iommu/iova: Fix module config properly

iommu/iova: Fix module config properly

IOMMU_IOVA is intended to be an optional library for users to select as
and when they desire. Since it can be a module now, this means that
built-in code which has chosen not to select it should not fail to link
if it happens to have selected as a module by someone else. Replace
IS_ENABLED() with IS_REACHABLE() to do the right thing.

CC: Thierry Reding <thierry.reding@gmail.com>
Reported-by: John Garry <john.garry@huawei.com>
Fixes: 1e1a22c65c75 ("iommu: Make the iova library a module")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/548c2f683ca379aface59639a8f0cccc3a1ac050.1663069227.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>