e1000: Clean up igb_txrx

The intention here is to reduce differences between em, igb, igc, ixgbe.

The main functional change is logical simplification in igb_rx_checksum
and getting interface caps from scctx instead of the ifp.

Reviewed by: gallatin, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30073

(cherry picked from commit 41f02257146d128cdd4aa2bdcfdab7f9e08a4314)