Input: xpad - clarify LED enumeration

Rename led_no -> pad_nr: the number stored there is not the LED number - it
gets translated later on to a LED number in xpad_identify_controller;

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