ethtool: fec: sanitize ethtool_fecparam->fec

ethtool: fec: sanitize ethtool_fecparam->fec

Reject NONE on set, this mode means device does not support
FEC so it's a little out of place in the set interface.

This should be safe to do - user space ethtool does not allow
the use of NONE on set. A few drivers treat it the same as OFF,
but none use it instead of OFF.

Similarly reject an empty FEC mask. The common user space tool
will not send such requests and most drivers correctly reject
it already.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>