net: phy: mxl-gpy: implement SGMII in-band configuration

net: phy: mxl-gpy: implement SGMII in-band configuration

SGMII in-band autonegotiation was previously kept untouched (and restored
after switching back from 2500Base-X to SGMII). Now that the kernel offers
a way to announce in-band capabilities and nable/disable in-band AN,
implement the .inband_caps and .config_inband driver ops.
This moves the responsibility to configure SGMII in-band AN from the PHY
driver to phylink.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/70f07e46dd96e239a9711e6073e8c04c1d8672d4.1767800226.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>