USB: cdc-wdm: fix wdm_find_device* return value

A logic error made the wdm_find_device* functions
return a bogus pointer into static data instead of
the intended NULL no matching device was found.

Cc: stable <stable@vger.kernel.org> # v3.4+
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>