i3c: dw: Return the length from a read priv_xfer

i3c: dw: Return the length from a read priv_xfer

We currently assume that the rx_len of a read command will be as
submitted, but we may have a shorter read than expected.

This change populates the output i3c xfer length from the actually-read
length.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/f4fff7ab18dee1f662dc7a5a4111fcd921e6792b.1680156630.git.jk@codeconstruct.com.au
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>