iommu/mediatek: fix use-after-free on probe deferral

In the Linux kernel, the following vulnerability has been resolved:

iommu/mediatek: fix use-after-free on probe deferral

The driver is dropping the references taken to the larb devices during

probe after successful lookup as well as on errors. This can

potentially lead to a use-after-free in case a larb device has not yet

been bound to its driver so that the iommu driver probe defers.

Fix this by keeping the references as expected while the iommu driver is

bound.