board: stm32pm1: Fix board_check_usb_power()

Depending of plugged power source (computer, wall charger, ...) it can
happen that we got the following message:

"****************************************************"
"*      USB TYPE-C charger not compliant with       *"
"*                   specification                  *"
"****************************************************"
"                                                    "
"### ERROR ### Please RESET the board ###            "

This issue has been detected on STM32MP135f-DK board.

It's due to max_uV and min_uV value are initialized at beginning of
board_check_usb_power() and can then be used for the 2 iteration of
adc_measurement().
In some cases, max_uV/min_uV values issued of the first adc_measurement()
iteration are used as input of the second adc_measurement() iteration,
which can lead to incoherent pair of min_uV/max_uV values.

To ensure that adc_measurement() returns coherent value for max_uV and
min_uV, initialize max_uV and min_uV at each loop start.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Change-Id: I37a715d0e8614c0ca5658cce21d5034bfaae6393
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/558130
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>

Author: Patrice Chotard

board/st/stm32mp1/stm32mp1.c

@@ -372,8 +372,8 @@ static int adc_measurement(ofnode node, int adc_count, int *min_uV, int *max_uV)
 static int board_check_usb_power(void)
 {
 	ofnode node;
-	int max_uV = 0;
-	int min_uV = USB_START_HIGH_THRESHOLD_UV;
+	int max_uV;
+	int min_uV;
 	int adc_count, ret;
 	u32 nb_blink;
 	u8 i;
@@ -401,6 +401,9 @@ static int board_check_usb_power(void)
 
 	/* perform maximum of 2 ADC measurements to detect power supply current */
 	for (i = 0; i < 2; i++) {
+		max_uV = 0;
+		min_uV = USB_START_HIGH_THRESHOLD_UV;
+
 		ret = adc_measurement(node, adc_count, &min_uV, &max_uV);
 		if (ret)
 			return ret;