drivers: i3c: check addr slot is free only if it's different

The bus helpers for SETDASA and SETNEWDA could have the same
dynamic address assigned as it's static addr for SETDASA or
as it's dynamic addr for SETNEWDA. (Althrough, only a clown
would set the new dynamic address to the same old dynamic
address, it is still a valid address to set to).

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>