So far, we blindly assumed that the all usb-audio mixer elements
follow the standard and apply the standard resume method for the
registered elements in the id_elems[] list.  However, some quirks
really need the own resume and it's incomplete for now.

This patch enhances the resume handling in two folds:
- split some fields in struct usb_mixer_elem_info into a smaller
  header struct (usb_mixer_elem_list) for keeping the minimal
  information in the linked-list; the usb_mixer_elem_info embeds this
  header struct instead
- add resume and dump callbacks to usb_mixer_elem_list struct to allow
  quirks providing the own methods

For the standard mixer elements, these new callbacks are set to the
standard ones as default, thus there is no functional change by this
patch yet.

The dump and resume callbacks are typedef'ed for ease of later patches
using arrays of such function pointers.
Signed-off-by: Takashi Iwai <tiwai@suse.de>