usb: gadget: uvc: configfs: Add bFormatIndex attributes

The UVC format description are numbered using the descriptor's
bFormatIndex field. The index is used in UVC requests, and is thus
needed to handle requests in userspace. Make it dynamically discoverable
by exposing it in a bFormatIndex configfs attribute of the uncompressed
and mjpeg format config items.

The bFormatIndex value exposed through the attribute is stored in the
config item private data. However, that value is never set: the driver
instead computes the bFormatIndex value when linking the stream class
header in the configfs hierarchy and stores it directly in the class
descriptors in a separate structure. In order to expose the value
through the configfs attribute, store it in the config item private data
as well. This results in a small code simplification.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/ABI/testing/configfs-usb-gadget-uvc

@@ -168,6 +168,10 @@ Description:	Specific MJPEG format descriptors
 
 		All attributes read only,
 		except bmaControls and bDefaultFrameIndex:
+		bFormatIndex		- unique id for this format descriptor;
+					only defined after parent header is
+					linked into the streaming class;
+					read-only
 		bmaControls		- this format's data for bmaControls in
 					the streaming header
 		bmInterfaceFlags	- specifies interlace information,
@@ -212,6 +216,10 @@ Date:		Dec 2014
 KernelVersion:	4.0
 Description:	Specific uncompressed format descriptors
 
+		bFormatIndex		- unique id for this format descriptor;
+					only defined after parent header is
+					linked into the streaming class;
+					read-only
 		bmaControls		- this format's data for bmaControls in
 					the streaming header
 		bmInterfaceFlags	- specifies interlace information,