usb: gadget: uvc: verify descriptors presence

If the caller of uvc_alloc() does not provide enough
descriptors, binding the function should fail, so appropriate
code is returned from uvc_copy_descriptors().

uvc_function_bind() is modified accordingly to account for possible
errors from uvc_copy_descriptors().

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>