firmware: Avoid manual device_create_file() calls

Use the static attribute groups assigned to the device instead of
manual device_create_file() & co calls.  It simplifies the code and
can avoid possible races, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>