USB gadget: Handle endpoint requests at the function level

Control requests targeted at an endpoint (that is sent to EP0 but
specifying the target endpoint address in wIndex) are dispatched to the
current configuration's setup callback, requiring all gadget drivers to
dispatch the requests to the correct function driver.

To avoid this, record which endpoints are used by each function in the
composite driver SET CONFIGURATION handler and dispatch requests
targeted at endpoints to the correct function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>