Input: mousedev - correct lockdep annotation

When annotating mutex to avoid false lockdep reports we should not
be using MOUSEDEV_MIX as lock subclass but rather SINGLE_DEPTH_NESTING.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>