MX300 does not have an EXTRA_BTN - it is a simple wheel mouse with
an additional task-switcher button, which is reported as side button
(and not task button).

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Adrian Bunk <bunk@stusta.de>