ethtool: Set cmd field in ETHTOOL_GLINKSETTINGS response to wrong nwords

When the ETHTOOL_GLINKSETTINGS implementation finds that userland is
using the wrong number of words of link mode bitmaps (or is trying to
find out the right numbers) it sets the cmd field to 0 in the response
structure.

This is inconsistent with the implementation of every other ethtool
command, so let's remove that inconsistency before it gets into a
stable release.

Fixes: 3f1ac7a7 ("net: ethtool: add new ETHTOOL_xLINKSETTINGS API")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>