platform/chrome: cros_ec: Separate logic for getting panic info

Create a separate function called cros_ec_get_panicinfo for getting
panic info from EC.

Currently cros_ec_create_panicinfo is the only caller.

Signed-off-by: Rob Barnes <robbarnes@google.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20230410165817.932449-1-robbarnes@google.com