platform/chrome: cros_ec_uart: fix race condition

serdev_device_set_client_ops() is called before `ec_dev` is fully
initialized.  This can result in cros_ec_uart_rx_bytes() being called
while `ec_dev` is still not initialized, resulting in a kernel panic.

Call serdev_device_set_client_ops() after `ec_dev` is initialized.

Fixes: 04a8bdd1 ("platform/chrome: cros_ec_uart: Add transport layer")
Signed-off-by: Robert Zieba <robertzieba@google.com>
[tzungbi: modified commit message and fixed context conflict.]
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20221229094738.2304044-1-tzungbi@kernel.org