hwmon: (ams) Fix permissions on 'joystick' module parameter

We should not allow writes to the 'joystick' module parameters since
writing there will not trigger creation of the input device. Disable
writes since we provide alternative way of enabling input device via
AMS device's sysfs attribute.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>