nfp: use the correct index for link speed table

sts variable is holding link speed as well as state.  We should
be using ls to index into ls_to_ethtool.

Fixes: 265aeb51 ("nfp: add support for .get_link_ksettings()")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>