hash-funcs: tweak odering in devt_compare_func()

hash-funcs: tweak odering in devt_compare_func()

Let's order dev_t's by their major first, minor secondary. The binary
encoding of the two fields is weirdly interleaved and different in
kernel and glibc, hence let's focus on the generic part that works like
users would expect it.

So far the function is only used to compare for equality, not for
sorting, hence this has no immediate effect.