net: ethtool: Add macro to clear a link mode setting

There are currently macros to set and test an ETHTOOL_LINK_MODE_ setting,
but not to clear one. Add a macro to clear an ETHTOOL_LINK_MODE_ setting.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>