Input: polled device - do not start polling if interval is zero

If the poll interval is set to 0 via new sysfs entry and device is
opened after that, polling is started with interval 0. This causes
huge CPU load. Same happens if the rate was 0 when the device was
closed and then reopened.

Signed-off-by: Samu Onkalo <samu.p.onkalo@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>