sndioctl: Remove assert about duplicate controls

sndioctl: Remove assert about duplicate controls

On the sndiod(8) side device controls are not ordered. While switching
from one device to another, a new control (of the new device) may
appear before an old control with the same name is removed. As
discussed in sioctl_open(3), once the full description increment is
fetched (i.e. the call-back is invoked with NULL sioctl_desc
structure) the representation of the control set is consistent.