net: ethernet: stmmac: fix altr_tse_pcs function when using a fixed-link

In the Linux kernel, the following vulnerability has been resolved: net: ethernet: stmmac: fix altr_tse_pcs function when using a fixed-link When using a fixed-link, the altr_tse_pcs driver crashes due to null-pointer dereference as no phy_device is provided to tse_pcs_fix_mac_speed function. Fix this by adding a check for phy_dev before calling the tse_pcs_fix_mac_speed() function. Also clean up the tse_pcs_fix_mac_speed function a bit. There is no need to check for splitter_base and sgmii_adapter_base because the driver will fail if these 2 variables are not derived from the device tree.