ALSA: usb: mixer: make string parsing independent of mixer_build state

ALSA: usb: mixer: make string parsing independent of mixer_build state

Functions like snd_usb_copy_string_desc() or
get_term_name() don't actually need mixer_build
state but can use snd_usb_audio structure instead
to get usb device.

This patch has no functional change but prepares
to future UAC3 BADD profiles support which don't
have class-specific descriptors so won't have
mixer_build state.

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>