stm32l1: rcc/lcd: fix RTCSEL HSE definition

Wrong since original commit in 2013.

LCD code can't actually automatically determine clock speed if it's HSE,
as we don't know here whether what HSE is, nor what it's divided by.
For more fun, that old 2014 API doesn't have any way of flagging that it
failed either.  Hooray.

lib/stm32/l1/lcd.c

@@ -131,9 +131,9 @@ void lcd_set_refresh_frequency(uint32_t frequency)
 	case RCC_CSR_RTCSEL_LSI:
 		lcd_clock = 37000;
 		break;
-	case RCC_CSR_RTCSEL_HSI:
-		lcd_clock = 16000000;
-		break;
+	case RCC_CSR_RTCSEL_HSE:
+		/* no current method of determining clock and divider! */
+		return;
 	default:
 		/* RCC Clock not selected */
 		return;