Dynamic SGMII PHY links may remain software-forced, which prevents PCS
state changes from generating MAC link interrupts. Enable phylink PCS
polling for in-band SGMII so links recover after the PCS settles
following a mode change.
Link: https://github.com/openwrt/openwrt/pull/25118
Signed-off-by: Jonas Jelonek <jonas@jonasjelonek.de>