platform/chrome: Use proper protocol transfer function

pkt_xfer should be used for protocol v3, and cmd_xfer otherwise. We had
one instance of these functions correct, but not the second, fall-back
case. We use the fall-back only when the first command returns an
IN_PROGRESS status, which is only used on some EC firmwares where we
don't want to constantly poll the bus, but instead back off and
sleep/retry for a little while.

Fixes: 2c7589af

 ("mfd: cros_ec: add proto v3 skeleton")
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Benson Leung <bleung@chromium.org>