Input: xpad - use ida() for finding the pad_nr

The pad_nr corresponds to the lit up LED on the controller. Therefore there
should be no gaps when enumerating. Currently a LED is only re-assigned
after a controller is re-connected 4 times.

This patch uses ida to track connected pads - this way we can re-assign
freed up pad number immediately.

Consider the following case:
1. pad A is connected and gets pad_nr = 0
2. pad B is connected and gets pad_nr = 1
3. pad A is disconnected
4. pad A is connected again

using ida_simple_get() controller A now correctly gets pad_nr = 0 again.

Signed-off-by: Pavel Rojtberg <rojtberg@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>