ethtool: fec: sanitize ethtool_fecparam->reserved

struct ethtool_fecparam::reserved is never looked at by the core.
Make sure it's actually 0. Unfortunately we can't return an error
because old ethtool doesn't zero-initialize the structure for SET.
On GET we can be more verbose, there are no in tree (ab)users.

Fix up the kdoc on the structure. Remove the mention of FEC
bypass. Seems like a niche thing to configure in the first
place.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>