fsi: Convert misc devs to proper chardevs and more

This converts the various FSI devices from misc dev to chardev, as there can potentially be too much of them for misc devs limited minors, and because there are some lifetime issues with the current support.

This provide a common infrastructure to allocate an FSI major and distribute minors in a way that keeps it compatible with existing userspace. A new representation grouping FSI devices under a /dev/fsi directory is optinally provided, which will work in conjunction with new udev scripts aimed at providing fixed ID based symlinks.

A side effect of those conversions is to fix some object lifetime issues caused by a mixup between devm_kzalloc and proper object lifetime.

This series also adds a /dev{/fsi}/cfamN chardev for raw CFAM access that will superseed the existing "raw" sysfs file.

Finally there's also a locking fix to avoid horrible mixups if the "rescan" file is poked while a rescan is already in progress.