driver core: firmware_class: convert to use class_groups

Convert the firmware core to use class_groups instead of class_attrs as
that's the correct way to handle lists of class attribute files.

Cc: Ming Lei <ming.lei@canonical.com>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>