platform/chrome: cros_ec_proto: separate cros_ec_xfer_command()

cros_ec_send_command() has extra logic to handle EC_RES_IN_PROGRESS.
Separate the command transfer part into cros_ec_xfer_command() so
that other functions can re-use it.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-6-tzungbi@kernel.org