linux: Don't overwrite guesses from previous siblings with UP_DEVICE_KIND_KEYBOARD

If a power supply has multiple siblings, any guesses to device type from previous siblings will be overwritten by a sibling with no guess, as it defaults to keyboard, the highest priority. Instead, while guessing, use UNKNOWN, and only default to keyboard at the end.

Could also be fixed by changing the keyboard's priority, but that might have side effects for otherwise working devices

Closes #221 (closed)