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>

include/linux/hid.h

@@ -342,6 +342,7 @@ struct hid_collection {
 struct hid_usage {
 	unsigned  hid;			/* hid usage code */
 	unsigned  collection_index;	/* index into collection array */
+	unsigned  usage_index;		/* index into usage array */
 	/* hidinput data */
 	__u16     code;			/* input driver code */
 	__u8      type;			/* input driver type */