Input: add input_get_poll_interval()

Some drivers need to be able to know the current polling interval for
devices working in polling mode, let's allow them fetching it.

Acked-By: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>