Bluetooth: Audio: Clear received_base in audio shell on PA sync lost

When the PA sync is lost, we clear the received_base so
that once we resync to the PA, we get the BASE again.

This is to make it easier to decode and use the BASE again
if the sync was lost, as the previous BASE may have
been printed a long time before.

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