ethtool: reformat kerneldoc for struct ethtool_link_settings

The kernel doc comments for struct ethtool_link_settings includes
documentation for three fields that were never present there, leading to
these docs-build warnings:

  ./include/uapi/linux/ethtool.h:2207: warning: Excess struct member 'supported' description in 'ethtool_link_settings'
  ./include/uapi/linux/ethtool.h:2207: warning: Excess struct member 'advertising' description in 'ethtool_link_settings'
  ./include/uapi/linux/ethtool.h:2207: warning: Excess struct member 'lp_advertising' description in 'ethtool_link_settings'

Remove the entries to make the warnings go away.  There was some
information there on how data in >link_mode_masks is formatted; move that
to the body of the comment to preserve it.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>