media: uvcvideo: Factor out usb_string() calls

When parsing UVC descriptors to instantiate entities, the driver calls
usb_string() to retrieve the entity name from the device, and falls back
to a default name if the string can't be retrieved. This code pattern
occurs multiple times. Factor it out to a separate helper function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>