platform/x86: dell-wmi-sysman: fw_attr_inuse can be static

drivers/platform/x86/firmware_attributes_class.c:11:5: warning: symbol 'fw_attr_inuse' was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20210603153936.GA65404@7832cb195c0b


Signed-off-by: Hans de Goede <hdegoede@redhat.com>