For igb(4), when we are doing multiqueue, we are all setup to have full 32bit
RSS hash from the card. We do not need to hide that under "ifdef RSS" and should
expose that by default so others like lagg(4) can use that and avoid hashing the
traffic by themselves.
While here, improve comments and get rid of hidden/unimplemented RSS support
code for UDP.

Differential Revision: https://reviews.freebsd.org/D2296
Reviewed by: jfv, erj
Discussed with: adrian
Sponsored by: Limelight Networks