usb: fix request hijack through Audio class workaround

Because it is not always possible to determine whether
a request is for Audio class or not, all requests are passed
to it. This can lead to the requests, to e.g. HID interface,
being hijacked by Audio class.
Ignore return value of Audio custom handler as it is not relevant here.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>