mgmt: mcumgr: Fix wrongly using pointer of pointer in free function
mgmt: mcumgr: Fix wrongly using pointer of pointer in free function

Fixes an issue introduced when the mcumgr code was simplified whereby
the newer compressed free function call wrongly passes a pointer to a
pointer instead of the pointer itself.

Signed-off-by: Jamie McCrae <7c977670495befe33b67661379bf81cc6b3660ab@nordicsemi.no>

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