firmware: Refactoring for splitting user-mode helper code

Since 3.7 kernel, the firmware loader can read the firmware files
directly, and the traditional user-mode helper is invoked only as a
fallback.  This seems working pretty well, and the next step would be
to reduce the redundant user-mode helper stuff in future.

This patch is a preparation for that: refactor the code for splitting
user-mode helper stuff more easily.  No functional change.

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>