HID: intel_ish-hid: various cleanups

I ran into a warning message during randconfig testing and spent way
too much time figuring out how to best address it. One thing led to
another and I ended up with a 5 patch series.

Unforunately I screwed up the first version of the series and
had to replace the first patch, but the second version should
be much better.

Please have a look at the first patch separately, it might fix an
important bug and need backporting to stable kernels, or it might
only address a harmless warning.

For the rest of the patches, please merge for 4.13 unless you
see something wrong.