BlueZ API as BAP Server gives us the ISO interval, instead of the SDU
interval, in the MediaTransport.QoS.Interval property.  They are not
necessarily the same. What we need is the SDU interval.

The SDU interval is the interval between packets the encoder outputs, so
it is determined by the codec configuration, and for LC3 is equal to the
frame duration.

Add codec method get_interval() that returns the correct interval, and
use it in iso-io.