hikey960: fix the calculation in boardid

Since the type of ADC value is always unsigned int, don't
need to check the value with negative value.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
diff --git a/plat/hisilicon/hikey960/hikey960_boardid.c b/plat/hisilicon/hikey960/hikey960_boardid.c
index ac3e0385..90faa9d 100644
--- a/plat/hisilicon/hikey960/hikey960_boardid.c
+++ b/plat/hisilicon/hikey960/hikey960_boardid.c
@@ -111,9 +111,7 @@
 {
 	int	ret;
 
-	if (adcin_value < HKADC_DATA_GRADE0)
-		ret = BOARDID_UNKNOWN;
-	else if (adcin_value < HKADC_DATA_GRADE1)
+	if (adcin_value < HKADC_DATA_GRADE1)
 		ret = BOARDID_VALUE0;
 	else if (adcin_value < HKADC_DATA_GRADE2)
 		ret = BOARDID_VALUE1;