samples: Bluetooth: BAP: Source: 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_out" then refers to the data
we receive from the host, to be sent over BT, so the "out"
terminology still works for that.

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