nb/intel/sandybridge: Restore nominal Vref for current channel

After aggressive read training, program nominal Vref for the current
channel, not only channel 0. This simple mistake can easily degrade
memory margins, especially when running at high speed (overclocking).

Tested on Asus P8H61-M PRO, still boots.

Change-Id: I12630fe33c5c786c8ec131c45c27180c3887d354
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47680
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/northbridge/intel/sandybridge/raminit_common.c

@@ -2555,7 +2555,8 @@ static int discover_edges_write_real(ramctr_timing *ctrl, int channel, int slotr
 		}
 	}
 
-	MCHBAR32(GDCRTRAININGMOD_ch(0)) = 0;
+	/* Restore nominal Vref after training */
+	MCHBAR32(GDCRTRAININGMOD_ch(channel)) = 0;
 	printram("CPA\n");
 	return 0;
 }