ethtool: fec: fix FEC_NONE check

Dan points out we need to use the mask not the bit (which is 0).

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 42ce127d ("ethtool: fec: sanitize ethtool_fecparam->fec")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>