usb: gadget: composite: redirect setup requests

If there are setup requests not directed to an endpont or an interface,
current config's setup() has been attempted so far.
This patch, in case the above fails, adds code which tries the setup() of
configuration's function if there is only one function in the configuration.

This behavior is required to provide equivalent of gadget zero with configfs.

The gadget zero has a "config driver" for sourcesink, but all it does is
delegating the request to the function proper. So when the equivalent gadget
is set up with configfs it needs to handle requests directed to
"config driver", but with configfs it is not possible to specify
"config drivers".

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>