ALSA: usb-audio: Clean up mixer element list traverse

Introduce a new macro for iterating over mixer element list for
avoiding the open codes in many places.  Also the open-coded
container_of() and the forced cast to struct usb_mixer_elem_info are
replaced with another simple macro, too.

No functional changes but just readability improvement.

Signed-off-by: Takashi Iwai <tiwai@suse.de>