ethtool: Fix unwanted section breaks in kernel-doc

A colon almost unavoidably starts a new section.  The script should be
changed to provide a way to avoid this, but for now reword the
comments to avoid using colons.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>