setupapi: Fix uninitialized len in is_device_instance_linked, use RegGetValueW, and check for refstr

[xzn]

Upstream wine bug: N/A. is_device_instance_linked in devinst.c is Valve's wine only it seems.

This pr does three changes:

1. Fix uninitialize len before the first RegGetValueW call (which was RegQueryValueExW before the change).
2. Set len correctly for RegGetValueW/RegQueryValueExW, both of which expect len to be size in bytes instead of count in chars (the case for RegEnumKeyExW).
3. Check for refstrs and return true if any of them is linked.