gpu: ipu-v3: csi: pass back mbus_code_to_bus_cfg error codes

mbus_code_to_bus_cfg() can fail on unknown mbus codes; pass back the
error to the caller.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
[p.zabel@pengutronix.de - renamed rc to ret for consistency]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>