bq27000: report missing device better.

One an hdq buss, a missing device reads as 0xff, not -1.
So do a translation to allow detecting of a missing bus.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/power/bq27x00_battery.c

@@ -416,6 +416,9 @@ static void bq27x00_update(struct bq27x00_device_info *di)
 	bool is_bq27425 = di->chip == BQ27425;
 
 	cache.flags = bq27x00_read(di, BQ27x00_REG_FLAGS, !is_bq27500);
+	if ((cache.flags & 0xff) == 0xff)
+		/* read error */
+		cache.flags = -1;
 	if (cache.flags >= 0) {
 		if (!is_bq27500 && !is_bq27425
 				&& (cache.flags & BQ27000_FLAG_CI)) {