hwmon: (acpi_power_meter) Fix compiler warning seen in some configurations

In some configurations, BUG() does not result in an endless loop but returns
to the caller. This results in the following compiler warning:

drivers/hwmon/acpi_power_meter.c: In function 'show_str':
drivers/hwmon/acpi_power_meter.c:380: warning: 'val' may be used uninitialized in this function

Fix the warning by setting val to an empty string after BUG().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>