i3c: master: svc: remove redundant assignment to cmd->read_len

The assignment of xfer_len to cmd->read_len appears to be redundant
as the next statement re-assigns the value 0 to it.  Clean up the
code by removing the redundant first assignment.

Addresses-Coverity: ("Unused value")
Fixes: dd3c5284 ("i3c: master: svc: Add Silvaco I3C master driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210224151349.202332-1-colin.king@canonical.com
(cherry picked from commit 436cb709)