kcm: Fix speaker test layout for Pro Audio profile

Known audio channels (like front-right, rear-left) are assigned rows 0 to 2 in the Grid layout so they are placed in their spatial position around the user's avatar

Unknown channels (those produced by the Pro Audio pipewire profile) have no spatial info and are assigned to rows 3 and following

The spacer items were also assigned also row 3, which collided with those, breaking the layout. Let's give the spacers a high enough row number so there is no collision with an already occupied cell

BEFORE	AFTER

BUG: 495752
FIXED-IN: 6.2.4