net: stmmac: Call xpcs_config_eee_mult_fact() only when xpcs is present

Some dwmac variants such as dwmac_socfpga don't use xpcs but lynx_pcs.

Don't call xpcs_config_eee_mult_fact() in this case, as this causes a
crash at init :

 Unable to handle kernel NULL pointer dereference at virtual address 00000039 when write

 [...]

 Call trace:
  xpcs_config_eee_mult_fact from stmmac_pcs_setup+0x40/0x10c
  stmmac_pcs_setup from stmmac_dvr_probe+0xc0c/0x1244
  stmmac_dvr_probe from socfpga_dwmac_probe+0x130/0x1bc
  socfpga_dwmac_probe from platform_probe+0x5c/0xb0

Fixes: 060fb27 ("net: stmmac: call xpcs_config_eee_mult_fact()")
Signed-off-by: Maxime Chevallier <[email protected]>
Reviewed-by: Russell King (Oracle) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: minimaxwell

drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c

@@ -524,7 +524,8 @@ int stmmac_pcs_setup(struct net_device *ndev)
 	if (ret)
 		return dev_err_probe(priv->device, ret, "No xPCS found\n");
 
-	xpcs_config_eee_mult_fact(xpcs, priv->plat->mult_fact_100ns);
+	if (xpcs)
+		xpcs_config_eee_mult_fact(xpcs, priv->plat->mult_fact_100ns);
 
 	priv->hw->xpcs = xpcs;