i3c: master: svc: return actual transfer data len

I3C allow devices early terminate data transfer. So set "actual_len" to
indicate how much data get by i3c_priv_xfer.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20231201222532.2431484-6-Frank.Li@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>