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: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>