usb: audio: correctly report internal delay

Every USB device must buffer isochronous data for at least 1 frame in
order to remove packet jitter within a frame. USB Specification Revision
2.0 describes necessary buffering in 5.12.5 Data Prebuffering. USB Audio
1.0 specification mentions buffering in 3.4 Inter Channel Synchronization.

Set bDelay to 1 instead of 0, because bDelay 1 is both the minimum
allowed value and matches the actual device behavior.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>