enetc: Remove Tx checksumming offload code

Tx checksumming has been defeatured and completely removed
from the h/w reference manual. Made a little cleanup for the
TSE case as this is complementary code.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Link: https://lore.kernel.org/r/20201103140213.3294-1-claudiu.manoil@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>