Merge branch 'ethtool-FEC'

Jakub Kicinski says:

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ethtool: clarify the ethtool FEC interface

Our FEC configuration interface is one of the more confusing.
It also lacks any error checking in the core. This certainly
shows in the varying implementations across the drivers.

Improve the documentation and add most basic checks. Sadly, it's
probably too late now to try to enforce much more uniformity.

Any thoughts & suggestions welcome. Next step is to add netlink
for FEC, then stats.
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Signed-off-by: David S. Miller <davem@davemloft.net>