firmware loader: embed device into firmware_priv structure

Both these structures have the same lifetime rules so instead of allocating
and managing them separately embed struct device into struct firmware_priv.
Also make sure to delete sysfs attributes ourselves instead of expecting
sysfs to clean up our mess.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>