igc: Add NVM/firmware prints and sysctl
This chipset suffered an (un)usual number of bugs and iterations. Let's
add our NVM/firmware code from e1000 and the similar igc_nvm function
from DPDK to keep track of issues.
Sponsored by: BBOX.io
(cherry picked from commit 33ed9bdca307bedb3d66a50ed7d4d7b4bf4acf39)