dinput: Explicitly define ASCII and Unicode methods in DirectInputDevice base class. Make ASCII methods call Unicode. Use new methods in child classes.

This reverts commit 061eb12c.
The patch makes tests for CredRead and CredEnumerate pass but breaks
CredWrite at the same time.

Its current implementation for XML_ENTITY_REF_NODE can't work and will 
always return VT_NULL variant anyways.