platform/chrome: cros_ec_typec: Factor out PD identity parsing

Factor out the PD identity parsing code into a separate function. This
way it can be re-used for Cable PD identity parsing in future patches.

No functional changes are introduced by this patch.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Benson Leung <bleung@chromium.org>
Link: https://lore.kernel.org/r/20201116201150.2919178-6-pmalani@chromium.org