enetc: Fix endianness issues for enetc_ethtool

These particular fields are specified in the H/W reference
manual as having network byte order format, so enforce big
endian annotation for them and clear the related sparse
warnings in the process.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>