Input: mousedev - signal that device is writable in mousedev_poll()

The Microsoft ImPS/2 mouse protocol being bidirectionnal (sic)
one may have to write in /dev/input/mice; and that works better
if select() does not hang.

Signed-off-by: Julien Moutinho <julm+linux@savines.alpes.fr.eu.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>