i3c: master: svc: rename read_len as actual_len

I3C transfer (SDR), target can early terminate read transfer.
I3C transfer (HDR), target can end write transfer.
I2C transfer, target can NACK write transfer.

'actual_len' is better name than 'read_len'.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20231201222532.2431484-5-Frank.Li@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>