LinuxKPI: ethtool.h add more definitions

While we do not currently use ethtool, add the definitions to avoid
other longer-term maintenance problems with drivers.

Also migrate ETH_GSTRING_LEN into here from if_ether.h as it seems this
is where it belongs.

Differential Revision: https://reviews.freebsd.org/D37214

(cherry picked from commit 1b91eba37b294fd191a9dc79a80f52495af9418d)