ethtool: reformat kerneldoc for struct ethtool_fec_stats

The kerneldoc comment for struct ethtool_fec_stats attempts to describe the
"total" and "lanes" fields of the ethtool_fec_stat substructure in a way
leading to these warnings:

  ./include/linux/ethtool.h:424: warning: Excess struct member 'lane' description in 'ethtool_fec_stats'
  ./include/linux/ethtool.h:424: warning: Excess struct member 'total' description in 'ethtool_fec_stats'

Reformat the comment to retain the information while eliminating the
warnings.

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