platform/chrome: cros_ec_proto: remove redundant NULL check

send_command() already checks if `ec_dev->pkt_xfer` is NULL.  Remove the
redundant check.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-6-tzungbi@kernel.org