platform/chrome: cros_ec_proto: Fix spelling mistake "unknwon" -> "unknown"

There is a spelling mistake in a dev_dbg message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220614064909.47804-1-colin.i.king@gmail.com