usb: pd: Correctly handle invalid Structured VDMs

As UFP since the policy engine only responds to the Discover
Identity command, all other commands are unrecognized and
therefore should be NAKed. Fix the handling as follows:

 - Ensure the standard Discover Identity was sent using
   SID of 0xFF00, otherwise it is invalid
 - Don't NAK incoming Attention messages
 - When sending a NAK, respond with the same SVID of the
   initiating command, whether it was correct or not
 - Enter Mode is not supported in general, but the NAK
   should also be sent back with the same mode index
 - Re-issue Discover SVIDs when more than 12 SVIDs are returned
 - Clean up macro name prefixes from 'VDM' to 'SVDM' to
   emphasize they are accessing structured VDM specific bits

Change-Id: Id2590065a01e46f8da70733d55f3bfabff5b532d
Signed-off-by: Jack Pham <jackp@codeaurora.org>