ALSA: usb: stream: move audioformat alloc/init into separate function

Offload snd_usb_parse_audio_interface() function which
became quite long after adding UAC3 spec support.

Move audioformat allocation and initialization
into separate function, this will make easier
future refactoring.
Attributes left in the original func because it'll
be used for UAC3 BADD profiles suport in the future

There is no functional change.

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