net: ethtool: don't ignore return from driver get_fecparam method

If ethtool_ops->get_fecparam returns an error, pass that error on to the
 user, rather than ignoring it.

Fixes: 1a5f3da2 ("net: ethtool: add support for forward error correction modes")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>