HID: add usage_index in struct hid_usage.

Currently, there is no way to know the index of the current field
in the .input_mapping and .event callbacks  when this field is inside
an array of HID fields.
This patch adds this index to the struct hid_usage so that this
information is available to input_mapping and event callbacks.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>