generic: pcs-mtk-lynxi: add hack to use 2500Base-X without AN

Using 2500Base-T SFP modules e.g. on the BananaPi R3 requires manually
disabling auto-negotiation, e.g. using ethtool. While a proper fix
using SFP quirks is being discussed upstream, bring a work-around to
restore user experience to what it was before the switch to the
dedicated SGMII PCS driver.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>