igc: Rename 'struct adapter' to 'struct igc_softc'
Rename the 'struct adapter' to 'struct igc_softc' to avoid type
ambiguity in things like kgdb and make sharing code with e1000 and
ixgbe easier.
Sponsored by: BBOX.io
(cherry picked from commit 542f5d56317e94676cf19cb9d66207bf9a6b6758)