greybus: battery: fix panic on operation error

If an operation times out or otherwise returns an error, val->intval
should not be set and an error-code should be returned.

Fixes a panic on unload while receiving -ENOTCONN.
Signed-off-by: Perry Hung <perry@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/battery.c

@@ -281,7 +281,7 @@ static int get_property(struct power_supply *b,
 		return -EINVAL;
 	}
 
-	return 0;
+	return (val->intval < 0) ? val->intval : 0;
 }
 
 // FIXME - verify this list, odds are some can be removed and others added.