power: supply: core: Fix boundary conditions in interpolation

The functions power_supply_temp2resist_simple and power_supply_ocv2cap_simple
handle boundary conditions incorrectly.
The change was introduced in a4585ba2050f460f749bbaf2b67bd56c41e30283
("power: supply: core: Use library interpolation").
There are two issues: First, the lines "high = i - 1" and "high = i" in ocv2cap
have the wrong order compared to temp2resist. As a consequence, ocv2cap
sets high=-1 if ocv>table[0].ocv, which causes an out-of-bounds read.
Second, the logic of temp2resist is also not correct.
Consider the case table[] = {{20, 100}, {10, 80}, {0, 60}}.
For temp=5, we expect a resistance of 70% by interpolation.
However, temp2resist sets high=low=2 and returns 60.

Cc: [e-mail]
Signed-off-by: Dorian Rudolph <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Fixes: a4585ba2050f ("power: supply: core: Use library interpolation")
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/supply/power_supply_core.c

···
 {
 	int i, high, low;
 
-	/* Break loop at table_len - 1 because that is the highest index */
-	for (i = 0; i < table_len - 1; i++)
+	for (i = 0; i < table_len; i++)
 		if (temp > table[i].temp)
 			break;
 
 	/* The library function will deal with high == low */
-	if ((i == 0) || (i == (table_len - 1)))
-		high = i;
+	if (i == 0)
+		high = low = i;
+	else if (i == table_len)
+		high = low = i - 1;
 	else
-		high = i - 1;
-	low = i;
+		high = (low = i) - 1;
 
 	return fixp_linear_interpolate(table[low].temp,
 				       table[low].resistance,
···
 {
 	int i, high, low;
 
-	/* Break loop at table_len - 1 because that is the highest index */
-	for (i = 0; i < table_len - 1; i++)
+	for (i = 0; i < table_len; i++)
 		if (ocv > table[i].ocv)
 			break;
 
 	/* The library function will deal with high == low */
-	if ((i == 0) || (i == (table_len - 1)))
-		high = i - 1;
+	if (i == 0)
+		high = low = i;
+	else if (i == table_len)
+		high = low = i - 1;
 	else
-		high = i; /* i.e. i == 0 */
-	low = i;
+		high = (low = i) - 1;
 
 	return fixp_linear_interpolate(table[low].ocv,
 				       table[low].capacity,