net: phylink: fix NULL pl->pcs dereference during phylink_pcs_poll_start

The current link mode of the phylink instance may not require an
attached PCS. However, phylink_major_config() unconditionally
dereferences this potentially NULL pointer when restarting the link poll
timer, which will panic the kernel.

Fix the problem by checking whether a PCS exists in phylink_pcs_poll_start(),
otherwise do nothing. The code prior to the blamed patch also only
looked at pcs->poll within an "if (pcs)" block.

Fixes: bfac8c490d ("net: phylink: disable PCS polling over major configuration")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Tested-by: Michael Walle <michael@walle.cc> # on kontron-kbox-a-230-ls
Tested-by: Nicolas Ferre <nicolas.ferre@microchip.com> # on sam9x60ek
Link: https://lore.kernel.org/r/20220629193358.4007923-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phylink.c

@@ -766,7 +766,7 @@ static void phylink_pcs_poll_stop(struct phylink *pl)
 
 static void phylink_pcs_poll_start(struct phylink *pl)
 {
-	if (pl->pcs->poll && pl->cfg_link_an_mode == MLO_AN_INBAND)
+	if (pl->pcs && pl->pcs->poll && pl->cfg_link_an_mode == MLO_AN_INBAND)
 		mod_timer(&pl->link_poll, jiffies + HZ);
 }