ixgbe: Improve device name strings
This is just clerical work to ease bug triage and may be used to set
expectations around the ability for anyone in the community to perform
testing and development on older parts.
Approved by: erj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D29876
(cherry picked from commit fdbcd35a75e596a78b4c0330eaded476eaa14797)