virtual_battery: Avoid unexpected batt_mode_cache refreshing

Currently we are assuming batt_mode would never be zero, but that is not
always true. Some battery do report zero for batt_mode(bob for example).

So everytime the batt_mode_cache been set to zero, the virtual_battery
would consider it uninited, and tries to refresh the next time.

Use -1 as uninited batt_mode_cache to avoid that.

BUG=b:66555246
BRANCH=gru
TEST=Check on bob, the battery level is correct.

Change-Id: Ieb7ec9403f69a6b5bca93c6682ec6117fe95fe1e
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-on: https://chromium-review.googlesource.com/678135
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>

common/virtual_battery.c

@@ -16,6 +16,8 @@
 #define CPUTS(outstr) cputs(CC_I2C, outstr)
 #define CPRINTS(format, args...) cprints(CC_I2C, format, ## args)
 
+#define BATT_MODE_UNINITIALIZED -1
+
 /*
  * The state machine used to parse smart battery command
  * to support virtual battery.
@@ -182,7 +184,7 @@ int virtual_battery_operation(const uint8_t *batt_cmd_head,
 	 * Note that we don't update the cached capacity: We do a real-time
 	 * conversion and return the converted values.
 	 */
-	static int batt_mode_cache;
+	static int batt_mode_cache = BATT_MODE_UNINITIALIZED;
 	const struct batt_params *curr_batt;
 	/*
 	 * Don't allow host reads into arbitrary memory space, most params
@@ -197,7 +199,7 @@ int virtual_battery_operation(const uint8_t *batt_cmd_head,
 			batt_mode_cache = batt_cmd_head[1] |
 					  (batt_cmd_head[2] << 8);
 		} else if (read_len > 0) {
-			if (batt_mode_cache == 0)
+			if (batt_mode_cache == BATT_MODE_UNINITIALIZED)
 				/*
 				 * Read the battery operational mode from
 				 * the battery to initialize batt_mode_cache.