firmware: Reduce ifdef CONFIG_FW_LOADER_USER_HELPER

By shuffling the code, reduce a few ifdefs in firmware_class.c.
Also, firmware_buf fmt field is changed to is_pages_buf boolean for
simplification.
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>