ixgbe: Clean up ix_txrx

The intention here is to reduce differences with D30072.
The only functional change is logical simplification in
ixgbe_rx_checksum.

Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D30074

(cherry picked from commit 51e46835e10a2628482deefc0efd356fb733337a)