samples: Bluetooth: BAP: Sink: Reverse in/out terminology

In USB "in" refers to data going from device to host,
and "out" refers to data going from host to device.

From a BT perspective "usb_in" then refers to the data
we send from the device to the host, that has been received
over BT.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>