usb_power_delivery: enforce numerical order of RDO/PDO flag annotations

usb_power_delivery: enforce numerical order of RDO/PDO flag annotations

Annotations of the USB power delivery decoder contain multiple text
fragments that correspond to several flags in bit fields. The Python
runtime did not guarantee an order of emission and made the test suite
fail.

Sort the order in which RDO and PDO flags related text fragments get
constructed and concatenated. Print text for higher bit positions first
as this might feel more intuitive to users.

This fixes part of bug #1090.