power_meter: acpi_device_class "power_meter_resource" too long

acpi_device_class can only be 19 characters and a NULL terminator.

The current code has a buffer overflow in acpi_power_meter_add():
       strcpy(acpi_device_class(device), ACPI_POWER_METER_CLASS);

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Len Brown <lenb@kernel.org>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>