net: stmmac: remove set but unused mask in stmmac_ethtool_set_link_ksettings()

This was never used since the commit that added it - e58bb43f
("stmmac: initial support to manage pcs modes").

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230406125412.48790-1-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>