ethtool: fec: sanitize ethtool_fecparam->active_fec

struct ethtool_fecparam::active_fec is a GET-only field,
all in-tree drivers correctly ignore it on SET. Clear
the field on SET to avoid any confusion. Again, we can't
reject non-zero now since ethtool user space does not
zero-init the param correctly.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>