Bluetooth: BAP: Shell: 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, and vice versa for
incoming BT data.

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