platform/chrome: cros_ec_typec: Correct alt mode index

Alt mode indices used by USB PD (Power Delivery) start with 1, not 0.

Update the alt mdoe registration code to factor this in to the alt mode
descriptor.

Fixes: de0f4948

 ("platform/chrome: cros_ec_typec: Register partner altmodes")
Signed-off-by: Prashant Malani <pmalani@chromium.org>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220819190807.1275937-3-pmalani@chromium.org