platform/chrome: cros_ec_proto: factor legacy out from cros_ec_prepare_tx()

cros_ec_prepare_tx() mixed the code for both versions.  To be neat and to
make it clear, factor the legacy part out as a separate function, rename
the function, and update the comments.

Specifically,
- prepare_tx(), for current protocol version (i.e. 3).
- prepare_tx_legacy(), for protocol version <= 2.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220518091814.2028579-3-tzungbi@kernel.org

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