mb/google/veyron_{mickey,rialto}: Remove return statement in void function

Return statement is not useful in void function.

Change-Id: I8cf020de335e4da933b7bbdc27b7ac6f31afe885
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81430
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/veyron_mickey/mainboard.c

@@ -92,5 +92,4 @@ void lb_board(struct lb_header *header)
 
 void mainboard_power_on_backlight(void)
 {
-	return;
 }

src/mainboard/google/veyron_rialto/mainboard.c

@@ -98,5 +98,4 @@ void lb_board(struct lb_header *header)
 
 void mainboard_power_on_backlight(void)
 {
-	return;
 }