driver-core: fix missing kernel-doc in firmware_class

Fix kernel-doc warning in firmware_class.c:

Warning(drivers/base/firmware_class.c:94): No description found for parameter 'attr'
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/base/firmware_class.c

@@ -78,6 +78,7 @@ firmware_timeout_show(struct class *class,
 /**
  * firmware_timeout_store - set number of seconds to wait for firmware
  * @class: device class pointer
+ * @attr: device attribute pointer
  * @buf: buffer to scan for timeout value
  * @count: number of bytes in @buf
  *