Fix egregious magic number constants

[shurd]

Summary

em and igb supported checksum offload bits were a hex constant.
Update to use lists of defined values.

[erj]

It's a reasonable change, but it's still kind of a surprise to me that em doesn't support IPv6 hardware offloads.

[sbruno]

In D17442#372066, @erj wrote:

It's a reasonable change, but it's still kind of a surprise to me that em doesn't support IPv6 hardware offloads.

I'm pretty sure that most of the modern hardware does, we just don't implement it. At least some of the chips did last time I looked.